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---------------------------------ASSISTED SCAVENGING TECHNOLOGY
By Tim Hickox
This paper will introduce a new class of engines, called FAST. They differ from existing engines in that the operating cycle is different. The function of a basic engine will be explained, then some alternative configurations will be offered. The advantages over traditional engines are: Simplicity; lower consumption of resources in manufacturing; lower first cost; lower operating costs; ease of repair (resulting in longer life); lower fuel consumption; low exhaust emissions; suitability for multi-fuel operation – particularly, hydrogen.
FAST is an acronym, short for Assisted Scavenging Technology with Fuel. AST can be used with a DFI (Direct Fuel Injection) system, e.g. Orbital, E-TEC. But an integrated DFI system is less complicated and more effective; in this form, AST becomes FAST. A FAST engine operates on the two-stroke, spark ignition (SI) cycle, meaning that there is a power impulse every 360-degrees of crankshaft rotation (for a single-cylinder engine). The engine uses conventional crankshaft, connecting-rod, piston, and bearing technology. Standard manufacturing processes and materials are used. Most of what has been learned about two-stroke engine design in the last 120 years can also be used in a FAST engine. FAST should be understood as technology that adds to current practice, rather than replacing it.
What FAST adds is not mechanical complexity, but a more conceptually complicated operating cycle. Much of the problem of introducing FAST follows an inadequate or false understanding of current two-stroke technology. “Education” tends to focus on dominate (in the marketplace) technologies; which may be practical, but implies an unreasonable correlation between sales and appropriate technology. It is a sad fact that manipulating a market can not only reduce the consumer’s options, it can determine what engineers may talk about. Consequently, the complexities of traditional two-stroke engines are not well understood by mechanical engineers, in general. Therefore, this paper will have to redress a history of development before introducing a more-appropriate technology.
Two-stroke engines have been around almost as long as four-stroke engines. George Brayton’s engine (patented in April 1872) was a “two-stroke” engine in the strict sense, but it is generally classified as a unique type. The “first two-stroke engine” is usually considered to be the design of Dugald Clerk, patented in 1879; three years after the (disputed) patent of Nicolaus Otto’s four-stroke engine. (In fact, Clerk’s engine bears a strong resemblance to Brayton’s engine.)
Clerk’s engine used a separate piston/cylinder as a pump to scavenge the working cylinder. This engine also had a poppet valve in the head. In comparison with Otto’s engine, Clerk’s appears more complicated and bulky. In a multitude of variations, the Clerk-type two-stroke survives today in CI (compression-ignition) engines. The largest and most efficient engines (0.260 lbs./hp/hr) in the world are Clerk-type two-strokes. A new company, Ecomotors, is building a factory in China to produce truck engines of this general type. About these engines we will say no more.
The complexity of the Clerk engine was eliminated by Frederick William Caswell Cock (1863-1944). He received a British patent (good for 14-years) in October 1892. (As usual, engines of Cock’s design were running years before the patent was issued.) Cock has been nearly forgotten; his engine is often credited to Joseph Day, with whose firm Cock was employed. Cock eliminated the separate pumping cylinder by using the crankcase as a pump; and he eliminated the valve(s) by having the piston uncover ports in the cylinder wall.
Cock’s design was clearly the simplest, cheapest engine ever conceived, with only three basic moving parts. And the lack of complexity resulted in the highest mechanical efficiency. Unfortunately, scavenging was problematic under light loads, resulting in misfiring and poor fuel consumption. Cock’s inspiration prevailed only where its virtues were highly valued and its vices tolerated. But it did survive, it evolved, and it has been the only common alternative to the small, four-stroke-cycle engine.
The first major change in the two-stroke’s evolution came in 1926, when Adolf Schnurle introduced a new arrangement of the ports, which improved trapping efficiency compared to traditional cross-scavenged engines. In the 1960s, the East German firm, MZ (formerly DKW) showed that a two-stroke engine’s exhaust system could be configured so that the pressure pulses worked like a pump on the exhaust side of the cylinder. A properly tuned exhaust system can increase peak power by about 50% over a straight pipe – a remarkable achievement that did not add any moving parts.
These major changes were accompanied by a multitude of small, detail refinements, particularly to the transfer ports. By 1980, small two-stroke engines were making more than 400-hp/liter. Motorcycle engineer, Kevin Cameron, said in 1981: “The single intractable fact remains: It is easier and cheaper to build fast two-stroke raceware than it is to build four-stroke. And why? Because the Brake Mean Effective Pressure (BMEP) of the four-stroke racing engine reached its apparent physical peak in the late 1940s. … When BMEP has come up to maximal levels, the only remaining path to power is higher RPM – a very expensive route. … The frontier of power in two-stroke engineering is not RPM but BMEP, which has crept up from 100-psi in the 1950s to 135-psi in the ‘60s. … [In 1981] it is closing in on four-stroke numbers.”
To appreciate what Cameron was saying, we need to look at the basic equation that predicts an engine’s power. It is (in one form):
Bhp = PLAN/33,000
P = pressure (BMEP)
L = piston stroke
A = piston area
N = RPM
Actually, this is the formula for a two-stroke engine. Since a four-stroke engine has only half as many firing cycles, at a given rpm, a small adjustment is required for four-strokes: the number “2” must be included somewhere. That is, the four-stroke can have twice the cylinder pressure, or twice the displacement, or it can turn twice the rpm. In practice, these three factors are usually balanced in some way, so that the four-stroke has more displacement, higher cylinder pressures, and turns higher rpm – in order to make the same power as a two-stroke. The four-stroke engine begins with a very serious disadvantage. To make the same power as a two-stroke, a four-stroke engine must resort to design options that make it more expensive and less efficient. That a two-stroke engine can produce the same power as a four-stroke with less displacement, lower cylinder pressures, and at lower rpm, means that the two-stroke is operating more efficiently. All of this is given by the basic equation.
Cameron was talking about racing engines, but the formula applies to all two- and four-stroke engines. And specific power output (the end-all in racing) is one measure of efficiency. Another measure is thermal efficiency. Unfortunately, the way “thermal efficiency” is commonly measured of engines produces an erroneous number in the case of many two-stroke engines. This error in measurement has given many persons the idea that the two-stroke cycle is inherently less efficient – the opposite of what we have just considered. To clear this up, the following test results are given. An introduction to these tests will be helpful. The information is taken from: Internal Combustion Engines, by Howard Degler (Professor of Mechanical Engineering, University of Texas).
In the first quarter of the 20th century, four-stroke gas engines were prevalent in industry. However, “… the oil fields used primitive two-stroke cycle [natural] gas engines … [which] were well constructed but they were by no means as economical of fuel as the four-stroke cycle engine …”. Initially, the oil fields had more natural gas than they could market (they burned off the excess), so fuel consumption was not an issue. As gas pressures dropped and markets for natural gas increased, more economical engines became a priority.
“The two-stroke gas engine had established itself as a reliable prime mover; however it became desirable to improve its fuel economy without the loss of its original advantages of simplicity, compactness, low first cost, and low maintenance. … [With the] standard two-stroke gas engine, part of the incoming gas always goes out through the exhaust port and is wasted. … By injecting the fuel under 10-15-psi, after the air has been injected and the ports closed, the gas consumption of the two-cycle engine has been reduced to that of the four-cycle engine …”.
These fuel-injected two-stroke engines went into service in the early 1930s, in sizes up to 300-hp, but only after the aforementioned tests. “In order to compare the gas consumption and general heat balance of a four-cycle and a two-cycle engine, with and without gas injection, tests were conducted on two engines that were identical in every respect except the power cylinder and camshaft mechanism.” When it ran with a carburetor, the two-stroke had poor fuel consumption, “… due to the large charging loss that is inherent in this type of engine, which in this instance runs as high as 30%.”
The first measured difference between two-stroke and four-stroke was a 50% power advantage for the two-stroke. But the four-stroke engine had adequate power for the application, so the two-stroke’s advantage could not be used in practice. However, it should be kept in mind that a smaller, more fuel-efficient two-stroke engine could be used in the practical application. The two-stroke, in this case, was oversized in order to keep both engine types as nearly identical as possible.
The engineers then proceeded to carefully measure the heat paths. This is the only test series that I know of where such measurements were taken of two- and four-stroke engines of like parameters. Some of the results are as follows:
HEAT LOSSES FOR TWO- AND FOUR-STROKE ENGINES AT 50% LOAD
Clearly, it is indisputable that the two-stroke engine’s thermal efficiency is much higher than the four-stroke’s. But this more efficient use of heat is never seen when engines are compared on the basis of “fuel consumption”. There has been a tacit refusal to recognize that fuel that never changes chemically cannot play a direct role in a thermodynamic process. Fuel that is lost before combustion is a problem, but it is not a thermodynamic problem. Failure to make this distinction has given many engineers a false picture of two-stroke cycle efficiency, and has resulted in much wasted effort aimed at improving two-stroke “efficiency”.
These test results should not surprise us; they are just what we might expect given the basic formula for power. We must now address another common misapplication of data that makes the four-stroke engine look better than it proves to be in practice. Brake Specific Fuel Consumption (BSFC) is usually given as the lowest figure measured. In four-stroke engines, this number is obtained at, or near, full-throttle and at a relatively high BMEP. But in many practical applications – road vehicles in particular – the engines very seldom operate in those conditions. Automobiles, for example, require a large power reserve for acceleration. At light loads, at cruising speeds, the engine is throttled and operates at the low end of the BMEP curve. BSFC curves rise rapidly as BMEP falls.
Tests by Ford engineers on various engines showed the lowest BSFC to be in the region of 0.45-lbs/hp/hr, which is typical of four-stroke SI engines. But in the BMEP range where these engines would be expected to operate most of the time, the BSFC was on the order of 0.60-lbs/hp/hr. Modern, carbureted two-stroke engines are not likely to approach the lower figure, but they can often match (or even better) the higher number under the same working conditions. Even though these simple two-strokes are losing a high percentage of fuel before combustion, their higher light-load thermal efficiency offsets those losses.
To put all this in perspective, we have to understand that the modern two-stroke’s advantages had been accepted and its disadvantages largely overcome by the 1960s. And this explains why four out of five motorcycles sold in the world were – at that time – powered by two-stroke engines. And from that point until recently, two-stroke engines completely dominated motorcycle racing. If you were in the four-stroke-engine business at this time, you could see that you were losing. Something would have to be done to make the four-stroke engine look better, or make the two-stroke engine look worse.
The one factor that stood out in the four-stroke’s favor was low hydrocarbon exhaust emissions. The person who used the engine was seldom aware of these, but the consequences to urban atmospheric conditions and “public health” were clear – that is, the air was not clear. Government restrictions on exhaust emissions became serious in the 1970s. The one card that the four-stroke-engine lobby had to play became a winning card.
By the late 1970s, when motorcycles in the U.S. fell under federal exhaust-emissions regulations, the Japanese companies had largely taken control of world motorcycle production. They were then in a commanding position to decide what measures should be taken to comply with the regulations. “They” decided (it appeared to be a national consensus) that the motorcycle industry should drop two-stroke engines and thereafter build only four-stroke engines. The reason was – ignoring all else – that two-stroke engines are “dirty” and four-stroke engines are “clean”. The one area where two-strokes continued to dominate was in racing, where exhaust emissions were never considered, and all other factors favored the two-stroke.
The argument against two-stroke motorcycle engines looked weak to anyone who followed the car industry. The “clean” four-stroke cycle was fighting for its life under the government’s regulated limits. At first, Detroit’s engineers just said, “We can’t do it!” When that failed to throw off the regulators, the industry went ahead and produced millions of engines that were very hard to start, stalled at intersections, overheated, and were worn out before they were paid for. Many people believed that if the government didn’t back down, the gasoline IC engine was doomed.
What saved the industry and the air were two new technologies: computer-controlled fuel injection, and the three-way catalyst. [The latter reduces hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). The previously used oxidizing catalyst did not lower NOx.] Someone might have wondered if the two-stroke could benefit from an injection of high-tech. Someone did, in Australia. A small group, the Orbital Engine Company, modified a three-cylinder Suzuki outboard engine. The most important change was a computer-controlled, direct-to-cylinder fuel-injection system (DFI).
The results were impressive. The Orbital group was convinced that the two-stroke would replace the four-stroke engine. They took out every patent that they could apply for and expected the world to beat a path to their door. Others were also impressed and began doing research into two-stroke engines. What they discovered is that an engineer who knows four-stroke engines gets lost inside a two-stroke. They are so different that little translates. When the engineers came back to say, “We don’t know how to do this”, their two-stroke programs were cut off and everybody went back to work on four-strokes.
As self-interest prevailed in the world, a few companies and individuals continued to work on two-strokes, because their self-interest was at stake. Outboard motors had almost always been two-stroke, and companies like Mercury and Evinrude had decades of experience designing and building two-stroke engines. Mercury adopted the Orbital injection system and continues to build two-stroke outboard motors up to 250-hp. Evinrude developed their own injectors, called E-TEC, and they continue to produce a full range of two-stroke outboards.
Although Honda was committed to making four-strokes, they continued through the ‘90s to make two-stroke engines for racing, because they couldn’t win races with four-strokes. So there were engineers at Honda who knew two-strokes and one of them was Yoichi Ishibashi, Chief Engineer, Honda R&D Co. Ltd., Asada, Japan. He knew that DFI could reduce the two-stroke’s emissions. But he had another idea.
Many years before, others had discovered that, under certain conditions, two-stroke engines exhibited an unusual combustion process. It was not initiated by a spark, and neither was it compression-ignition. The critical factor was chemical radicals left over from the previous cycle. Fuel, oxygen, and temperature also had to fall within an effective range. While the phenomenon had been studied, early researchers had concluded that the necessary conditions could only be sustained over a narrow speed/load range. Ishibashi showed that by throttling the exhaust port and using a computer to regulate the parameters, Active-Radical Combustion (ARC) could operate over almost the entire useful load range.
He also showed that ARC was more efficient than normal SI or CI combustion, resulting in lower fuel consumption and lower emissions. When combined with a DFI system, the two-stroke engine was a model of propriety. When his 400-cc, single-cylinder experimental engine was compared with one of Honda’s well-developed four-strokes, HC emissions were nearly the same, CO emissions were five-times higher for the four-stroke, and NOx emissions were seven times higher for the four-stroke. The two-stroke’s fuel consumption was 15% better. (These tests were run on the LA4 test cycle.) He concluded by calling the two-stroke “one of the most promising solutions for the environmental conscious power units.”
After proving the concept with the 400-cc engine, Ishibashi built a 250-cc single aimed specifically at lowering BSFC. The engine produced 20-kw (27-hp). On the European ECE-40 test cycle, it got 32-km/liter. A larger-displacement, production Honda four-stroke, also making 20-kw, got 19-km/liter on the same test. Another production Honda four-stroke, 250-cc and 13-kw, got 26-km/liter. After these tests, Honda management stopped all further two-stroke development.
Honda also stopped making two-stroke race engines after the rules were changed so that four-strokes would race with twice the displacement of two-strokes. The rest of the Japanese motorcycle industry followed Honda’s initiative.
Many other examples could be given, but the conclusion is that technical merit had nothing to do with decisions to make four-stroke engines, or decisions not to make two-stroke engines. On the contrary, there is a suggestion that two-stroke engines had become so superior to four-strokes that entrenched industries took extreme measures to protect vested interests.
ASSISTED SCAVENGING TECHNOLOGY
With certain antecedents in evidence, we can proceed to new technology. To say that the two-stroke cycle is superior to the four-stroke cycle, and to demonstrate this fact in engines built and tested, is not to say that current two-stroke engines have reached an evolutionary end (which some critics have suggested). While enormous amounts of money have gone into four-stroke development, and this continues, two-stroke resources have gone from little to nothing. For the well-intentioned two-stroke engineer, there is a huge amount of exciting work to be done that needs funding. AST is an example of what lies ahead.
The problem that faces the two-stroke designer is this: It is necessary to purge the cylinder of exhaust residuals from the previous cycle at the same time that a fresh charge of air is introduced. To do this, the crankcase, the cylinder, and the exhaust system must all be open concurrently. And, the exhaust system, as well as the crankcase and cylinder, must be considered a “pump”. What’s more, the timing of the exhaust port must be symmetrical around BDC (Bottom Dead Center), and since it must open well before the transfer ports, it must close that much after them. For a long time, most engineers did not take the two-stroke seriously, for they did not see how this process could be anything but hopelessly inefficient. But the process is much more complex than they could imagine or understand. What is at work here is synergy.
In comparison, the four-stroke cycle seems to be a model of reasonableness. On one stroke of the piston, air is drawn into the cylinder; on another stroke, the air is compressed; one more stroke pushes the piston down and rotates the crankshaft; finally, one stroke forces out the residual exhaust gas. Altogether, it forms a neat series and lends itself to cause-and-effect thinking. But one thing bothers – there seems to be a lot of preparation, and not much action. The two-stroke is all action, and this makes the process very hard to follow.
We have been talking about engines that use the crankcase as a primary pump. F.W.C. Cock used the crankcase because it is there, because it is free, and because it works. But if we look at it within the context of the whole scavenging process, it is far from ideal. If we assume that “scavenging” (the replacement of residual exhaust gas with fresh air) takes place when the transfer ports are open, the crankcase can only act as a pump during the first half of that cycle, before BDC. After BDC, the crankcase volume is increasing and the cylinder volume is decreasing. If gases did not have mass and momentum, we might even imagine that the negative work, after BDC, would cancel positive work, before BDC, and nothing at all would be accomplished! But gases do have momentum, and this radically changes that simplistic picture.
Likewise, we have to see that the gases going out the exhaust duct want to keep going and can pull the pressure in the cylinder below atmospheric pressure. But think about that statement. How can we speak of the “pressure in the cylinder” being different from the “pressure in the exhaust duct” (or in the crankcase), when there are large holes (the ports) interconnecting “them”? Are we talking about one “thing”, or two “things” or three “things”? What we have here is a dynamic system, continually changing. And the conceptual lines that we would like to draw between crankcase and cylinder, and between cylinder and exhaust system, cannot be valid. We have to understand that the density of the gases varies from place to place and these are “instantaneous pressure differences” that force gas movement. It is possible – and it has been done – to build an “engine” that runs without a piston, where scavenging of a combustion chamber is entirely dependent on pressure waves driving gas flow. If you can’t understand how such an “engine” can run, you will never understand how two-stroke engines work.
What we can say about using the crankcase as a primary pump is that its phase-relationship to the scavenging process is poor. When the transfer ports open, crankcase pressure will be highest, and it will then drop off very quickly. Most of the air will be delivered to the cylinder before BDC. And here is the problem: At “low speeds”, this air will move through the cylinder too quickly and as much as 40% will be lost out the exhaust. If that air is laden with fuel, a like percentage of unburned fuel will go out the exhaust.
Here is the not-thermodynamic problem to be solved. Ideally, the pressure at the transfer ports when they open should be minimal, and most of the air should be delivered near the end of the cycle, as the ports close. This would result in higher “trapping efficiency”, i.e. less air lost to the exhaust before the exhaust port closes.
That we need to slow down the scavenging process must come as a surprise to many, for it was long thought that the cylinder could not be adequately scavenged near BDC; that there wasn’t enough time. But if we look at a 100-cc kart engine that makes 13-kw at 16,000 rpm, we have to accept that the whole scavenging process can transpire very quickly. And we may then understand that at a “low speed”, like 6000-rpm, the process may run too quickly.
This explains why transfer port area has evolved ever upward; more area reducing flow velocity and slowing down the scavenging process. And in racing engines, even at 13,000-rpm, increasing crankcase volume (thus lowering pressure) has increased horsepower.
Another benefit of reducing flow velocity is that it results in less mixing of exhaust and air. Ideally, we want the air to displace the exhaust gases, not mix with them. Of course, mixing is inevitable, but the largest possible port area and lower nozzle velocity will produce less mixing.
All of this was understood by Manuel Sevilla Sanz, and he thought that he could improve on the traditional crankcase-scavenging process. He picked up an idea that DKW engineers had played with in the 1930s. In the early 1980s, he modified some engines and tested them. The results were impressive. We will look at the results of Sevilla’s tests because he laid the conceptual groundwork that led to AST.
What Sevilla wanted to do was to correct the phase relationship between crankcase pump and scavenging process. In the traditional engine, the one piston (which must be both pumping piston and working piston) reverses direction at BDC. If a second piston was used for pumping, we might connect the two of them so that the pumping piston did not reverse direction until the working piston was about 60-degrees past BDC. This relationship would be ideal, because the pumping piston would continue to pump air into the working cylinder throughout the entire scavenging process. This was, in effect, what Sevilla accomplished.
We will look first at torque charts for standard engines and for his modified engines. What makes these graphs so remarkable is that he did not change the transfer or exhaust port timing, or the combustion chamber/compression ratio, or the complete exhaust system. He changed only what was happening in the crankcase.
Torque charts of three engines.
The red line on the graph is the torque curve of the original 237-cc engine. The green line is a 349-cc engine of the same general type. The yellow line is the 237-cc engine that Sevilla modified. Clearly, changing the crankcase of the 237-cc engine had a much greater effect than a large increase in displacement. This tells us that we can get the same performance from smaller engines if we improve the primary pump’s charging characteristics – without changing the porting or exhaust system.
We are not concerned here with the actual physical modifications that Sevilla made. What we want to understand is what those changes did to the relationship between the crankcase and the cylinder, or what we might call the pumping characteristics. We can see this graphically by looking at the pressure/time history of both the crankcase and cylinder, at the transfer ports.
Pressure/time histories of standard and modified engines.
The red line here is the original 237-cc engine. The pressure rises in the crankcase (from left to right) until the transfer ports open. The pressure then drops very quickly. As Sevilla said, “Crankcase pressure has totally collapsed long before the ports have attained their maximum area.” Air will continue to flow into the cylinder due to momentum, but nothing is driving it. Near the end of the cycle, as the transfer ports are closing, we generally see some reverse flow, from cylinder to crankcase.
The yellow line is the modified engine. Crankcase pressure here is much lower when the ports open, only a little more than half the pressure of the original. And the gas velocity will be proportionately lower. As usual, the pressure drops rapidly as the ports open. But then the pressure rises again – quite unlike the original. We may summarize these two charts by saying that the original engine gives a single-phase cycle, beginning with high pressure; then the pressure falls off to nothing. The modified engine has two-phase scavenging; an initial lower-pressure burst that falls off rapidly, and then a second phase that is more prolonged.
The area under the curves is important, but the air that enters the cylinder late in the cycle is less likely to be lost because it has less time to travel the distance before the exhaust port closes. Even if the amount of air was the same in both cases (the same “delivery ratio”), the trapping efficiency would be better for the engine with late charging.
Although the result that Sevilla got, the very broad torque curve, is impressive, we don’t know what an ideal pressure/time history would look like. Nobody has done the work that would narrow down the possibilities. The history taken from Sevilla’s engine is the result of the architecture that he adopted, with practical implications, given a production engine to modify. It was not flexible. He could not easily vary parameters and test alternatives. Furthermore, his method of phase correction was bulky and lowered mechanical efficiency. It was an excellent test, but it was not a very practical design for a production engine.
What Sevilla proved without doubt is that the simplicity of using the crankcase as a primary pump comes at a price. There is a substantial performance improvement waiting to be exploited by a design change that remaps the pressure/time history. That said, the question that follows must be: How can we do that? One answer is AST.
The problem may be expressed thusly: We can continue to use the crankcase as a pump, but it will only work for … let’s call it Phase-one scavenging – before BDC. To get the results that Sevilla got, or better, we need a Phase-two – we need to push more air into the cylinder after BDC. We could use a separate pump, which would be phased to work in that period. That is, we could do much the same thing that Sevilla did, and we would then have to accept increased bulk, increased cost, lower mechanical efficiency, and probably some maintenance issues. The tradeoffs are not very attractive.
AST begins with the idea that the working piston has to do all the work, so that there is no loss of mechanical efficiency. This is one of the two-stroke’s advantages over other engines, so we don’t want to throw that away. What we need to do is to store some of that energy. Instead of dissipating it all in Phase-one, we can store some and use it in Phase two. The way to do this is to add a plenum, connected to the crankcase through reed valves (or any sort of valve that serves the same purpose). A plenum is just a closed volume, which is easily incorporated into the crankcase casting.
It works like this: As the piston falls, the pressure in the crankcase increases. The reed valves allow air to flow into the plenum, so plenum pressure follows crankcase pressure. When the transfer ports open, crankcase pressure falls rapidly, but plenum pressure remains high because the reed valves prevent reverse flow. The crankcase discharges into the cylinder, as usual, and here is Phase-one.
The energy in the plenum is now available for Phase-two scavenging, but we need some sort of valve to release the pressure at the right time – near BDC. One easy way to do this is to incorporate a peripheral valve into the magneto flywheel. This arrangement will not add any moving parts to the simple engine.
It is understood that the plenum will increase the volume of the “crankcase” (which now includes the plenum). This means that the primary compression ratio will be lower, and the peak pressure will be lower. This is exactly what we saw in Sevilla’s engine, and this is what we want. We want to reduce the velocity of the air and slow down the process. We want to impart less momentum to the air mass. It is that momentum that is forcing the air out the exhaust port – after the exhaust – and over-scavenging the cylinder.
As to the volume of the plenum, it should be at least equal to the cylinder’s displacement. A general rule is that a “low-speed” engine will work better with a larger plenum volume than a “high-speed” engine. Of course, all the variables have to be considered, and some of these will be mentioned later. A larger volume will result in lower initial pressure and a lower rate of change than a smaller volume.
We now have the means to do two-phase scavenging. But we are not going to follow Sevilla. His Phase-two was really just a continuation of Phase-one, because all the air was delivered through the same transfer ports. We can connect the plenum to different ports and gain a function that no two-stroke has had. This is how we are changing the operating cycle and rewriting the book. With AST, Phase-two is an entirely new operation.
Let’s review what happens in a traditional two-stroke engine, then see how we can improve on that. The exhaust port opens and the pressure in the cylinder drops to nearly atmospheric, but the cylinder is still full of residual exhaust gas. When the transfer ports open, the incoming air, from the crankcase, must force out and replace the exhaust gasses. Schnurle-type transfer ports direct the air to the rear of the cylinder (opposite the exhaust port). The air flows up the rear wall, around the cylinder head, and down to the exhaust port, in a loop-flow pattern. It took 100 years of development to get this right! We have to do this and we know how to do it well. But that air has momentum; and once we get it moving like this, it wants to keep going. With nothing to stop it, a large percentage (about 40%) of the air will go out the exhaust. And here is where the expansion chamber exhaust system comes to the rescue. A returning pressure wave forces much of that air back into the cylinder. It’s a nice effect, but it only works over a narrow, tuned-for, speed range. At “low” speeds (below that range), the air goes out and is lost. That means less torque. And if that air contains fuel, as in the typical carbureted two-stroke, that means high hydrocarbon emissions and poor fuel consumption. This is why we want to slow down the process and reduce those losses.
With AST, we can further reduce the quantity of air lost to the exhaust, and create a stratified field in the cylinder. We do this with special ports, connected to the plenum, through the valve. These ports are opposite the exhaust port, and direct the Phase-two airflow against the loop-pattern flow generated by the transfer ports. By BDC, the upper portion of the cylinder has been adequately scavenged, so we can stop that air that is heading for the exhaust port. We can create a more quiescent air mass, particularly in the upper half of the cylinder. The flow from the AS ports will form a sort of aerodynamic “wall”, separating the upper and lower halves of the cylinder. The relatively quiescent air in the upper half will be trapped, while the lower half will continue to fill with air from the transfer ports and AS ports. This additional filling will be from the rear of the cylinder to the front, rather than from top to bottom.
Although Assisted Scavenging can be used with an in-head DFI system (e.g. Orbital), the plenum permits the use of an integrated DFI system. What’s more, this new design can use a carburetor to meter the fuel and control the air/fuel ratio. For applications where first-cost is a critical factor, this allows us to build simple, inexpensive engines that are also clean and efficient. Where first-cost is less of an issue, and the lowest exhaust emissions or lowest fuel consumption is desired (or regulated), a computer-controlled solenoid-type fuel injector (as in four-stroke, port injection) can replace the carburetor. In either case, AST becomes FAST.
Combining a carburetor and “Direct Injection” is simple. The carburetor works exactly as it does in a normal two-stroke; that is, the carburetor is connected to the crankcase through a reed valve. The only difference is that the manifold is longer than usual. As the piston rises and crankcase volume increases, air is drawn through the carburetor and fuel is metered in proportion. A rich air/fuel mixture enters the manifold and is pulled toward the crankcase. But before the fuel can actually enter the crankcase, the piston passes TDC and the induction cycle stops. The manifold, not the crankcase, is now charged with an air/fuel mixture.
As the piston continues downward, it covers the inlet port (port ‘B’ in the illustration), then it uncovers the FAST port (port ‘A’ in the illustration) which is between the AS ports. Near BDC, the FAST valve opens and air from the plenum forces the mixture in the manifold through the FAST port. With the inlet to the crankcase closed by the piston, and the FAST port open, the mixture can only go into the cylinder. The nozzle of the FAST port aims the mixture at the sparkplug. The manifold is then cleared of fuel and ready for the next inlet cycle. No significant amount of fuel ever enters the crankcase.
The FAST valve opens before the AS valve (they are independent) so that the highest pressure in the plenum will drive the mixture through the FAST port to the sparkplug. It is essential to concentrate mixture around the sparkplug, or light-load misfiring will result – that is, one of the traditional problems will remain.
The AS valve opens shortly after the FAST valve and creates the “wall” that isolates this mixture around the sparkplug, preventing it from migrating away from the plug and toward the exhaust. With this design, very little fuel comes in contact with the cylinder wall, which aids lubrication and reduces hydrocarbon emissions. As the piston compresses the air, the mixture will become more nearly homogenous by the time the sparkplug fires, but there will be some stratification until the piston passes TDC and the turbulence generated by the squish area of the combustion chamber causes complete mixing.
This illustration shows how the carburetor is connected to the crankcase and how the FAST valve can then force the mixture into the cylinder. (This is only a schematic.)
Carburetor function in basic FAST engine.
The following illustrations show the AS and FAST valves, ports and in-cylinder flow patterns. (Keep in mind that the valves are side-by-side, but independent.)
AS valve, ports, and flow pattern. Green arrows are flow from crankcase through transfer ports, in traditional loop-pattern. Yellow arrows are air only (no fuel) from AS ports. The main inlet to the crankcase is not shown here.
Because the timing of the valve is easily altered, and the volume of the plenum as well, the pressure/time history can be quickly optimized. Phase-one and Phase-two scavenging are independent operations, so either can be changed without altering the other (to put it rather too simplistically – in reality, everything changes everything else). This complexity gives great freedom in determining the form of the pressure/time curve.
The FAST valve. Green arrows are airflow from traditional transfer ports. Yellow arrows are airflow from AS ports. Orange arrows are air/fuel mixture from FAST port to sparkplug. The reed valve at the port prevents blowback when cylinder pressure is high.
With the basic operation of a FAST engine explained, we can consider some options. You might notice from the illustrations that the FAST/AS ports are higher than the traditional transfer ports (which remain unaltered). They are in what has been called the “blow-down” region, where cylinder pressures are high, but dropping quickly as the exhaust port is uncovered. The FAST/AS ports thus substantially increase the area through which air enters the cylinder. And they can continue to deliver air to the cylinder after the traditional transfer ports have closed. Since they are not active until about BDC, their “timing” must be considered asymmetrical.
As described, the pressure in the plenum can never be higher than the pressure in the crankcase. In some cases it may be desirable, or even necessary, to raise plenum pressure above crankcase pressure. This is easily done. The AS ports are higher than the transfer ports; nearly as high as the exhaust port. When the AS ports are uncovered by the piston, the pressure in the cylinder is still relatively high. Normally, backflow is prevented because the valve is closed at this time. But if the valve is slightly open, cylinder pressure will force the air in the manifold (from the previous cycle) back into the plenum, increasing the pressure in the plenum. Altering the valve timing will control how much the pressure increases. This is called “backcharging”. When this is used, the manifold will become filled with exhaust gas. When the AS valve opens, near BDC, exhaust will come out first, clearing the manifold before air is discharged. This will have almost no effect on performance. Any gas, air or residual exhaust gas, can oppose the loop-flow stream and create the aerodynamic “wall”. The first air (if it be so) out of the port will be lost to the exhaust in any case, before the loop stream’s momentum can be arrested.
If the main air inlet to the crankcase is throttled, to control power output, the plenum will be throttled as well, since it is fed from the crankcase. Generally, we want to keep plenum pressure “high”, because it has to deliver the fuel mixture to the sparkplug. So, instead of throttling the crankcase, we can throttle the transfer ports. This will limit the amount of air that can enter the cylinder, and thus limit power, but crankcase pressure is unaffected. In this case, the main air inlet to the crankcase is open, something like a typical CI engine.
The AS and FAST ports are not throttled, so the transfer ports will be closed off completely at idle, and all of the air will then come through the AS and FAST ports. At peak BMEP (wide open throttle), about one-third of the total air mass delivered to the cylinder will come through the FAST/AS ports. The manifolds should be sized accordingly. At idle, 100% of the air will come through those ports, meaning that the scavenging process at this time is entirely different from a conventional two-stroke. This is necessary in order to concentrate a combustible mixture at the sparkplug and avoid the traditional misfiring.
If EFI is used, the long manifold between carburetor and crankcase is deleted, along with the associated inlet port. The fuel is then injected into the antechamber behind the reed valve which prevents blowback. To use a heavy fuel, e.g. diesel, an ultrasonic resonator is used in the antechamber, which breaks up the fuel, keeps it suspended in the air, and facilitates combustion. This option is particularly attractive to the military, which does not want to use gasoline, and wants lighter, quieter engines than CI can provide.
The exhaust system is unaffected by FAST and a tuned expansion chamber is used whenever space permits. Regardless of the exhaust system, a throttle can be incorporated into the exhaust port, near the piston. Throttling the exhaust will produce Active Radical Combustion as described in the “History” section.
If the highest specific power output is desired, a centrifugal compressor, driven by an electric motor, can charge the plenum through a reed valve. If plenum pressure is then kept above crankcase pressure, the crankcase will not charge the plenum, and crankcase pressure will be as high as if the plenum was not there. Plenum pressure will only be limited by the capacity of the centrifugal compressor and a blow-off valve. The beauty of this system is that the centrifugal compressor can be turned off at any time and the engine will then automatically go back to being a normal FAST engine, with no loss of mechanical efficiency. The engine then has two modes, power and economy. Obviously, EFI is required in this case in order to regulate the air/fuel mixture in accord with the mode in use.
An important option in the future will be the ability to use hydrogen fuel. The objective here requires some explanation. It has been known for a long time that thermal efficiency increases as the air/fuel ratio increases (goes leaner). (When comparing different fuels, air/fuel ratio is expressed as an equivalence ratio, where 1.0 is stoichiometric, a lower number is leaner, a higher number is richer.) A limit to improving thermal efficiency with hydrocarbon fuels results from the fuel’s failure to ignite or from incomplete combustion at equivalence ratios below about 0.65. Hydrogen is unique; compared to any hydrocarbon, it is very easy to ignite, even at ratios below 0.2. But this does not mean that we must use 100% hydrogen. As little as 4-5% of the energy may come from hydrogen, and the remainder from any hydrocarbon. The hydrogen will then ignite at a lean ratio and initiate combustion of the hydrocarbon as it goes. Using hydrogen in this way will result in an improvement in thermal efficiency on the order of 25%.
The usual objections to hydrogen lose significance when the amount of hydrogen used is very small. In automobiles, most of the hydrogen can be generated using braking energy in urban driving. The vehicle is slowed by an alternator, and the resulting electricity goes to an ultra-capacitor, then to an electrolyzer – sometimes called a reverse fuel-cell. Very little hydrogen need be stored, so safety is not an issue. No refueling is required so no fuel storage and delivery infrastructure is needed.
The hydrogen and the hydrocarbon fuel are injected into the antechamber by a single Orbital-type injector – which injects both a liquid and a gas (usually air, but in this case, hydrogen). In this application, lower pressures are required than in typical DFI applications. The two-stroke engine is particularly suited to hydrogen. Four-stroke engines operate at higher temperatures and the hot exhaust valve(s) cause pre-ignition on hydrogen. Four-stroke engines that run on hydrogen usually produce only about half as much power as the same engine running gasoline. Two-stroke engines usually produce about the same power on gasoline or hydrogen. It is therefore possible to achieve thermal efficiencies better than CI engines with simple, light, powerful SI two-stroke engines.
As said at the beginning, there is a lot of exciting work to be done in two-stroke engine development. Small companies with very limited R&D budgets have been making progress. Unmanned Aerial Vehicles (drones) have opened a new field where the SI two-stroke’s high power-to-weight ratio and its proven ability to burn aviation fuel (without high compression ratios) gives it advantages over all other options. But not just aviation fuel – these are true multi-fuel engines, and they are in production.
In motorcycles, a nearly industry-wide effort to eliminate two-stroke engines has failed. Four-stroke racing engines have proven to be too expensive and riders are now returning to two-strokes. Racing rules by the FIM and AMA that favor four-stroke engines are still a problem, but club racing has generally returned to the old rules that break classes according to displacement. One of the aims of the FAST development team is to see two-stroke road machines return to prominence.
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