Parallel Paths

2 Stroke Vs 4 Stroke

Does the introduction of the four-stroke powered Yamaha RX-1 mark the beginning of the end for two-stroke engine use in snowmobiles? While other manufacturers are expected to follow suit with their own versions of this highly successful formula, don’t be too quick to close the book on two-strokes. Two-stroke engines are alive and well in other markets where emissions are tightly regulated. As emission targets and requirements are finalized for the snowmobile industry, the snowmobile manufacturers are poised to provide more than one way to meet the goals. Expect “Parallel Paths” of development well into the future.

Today, in the glow of raves over Yamaha’s four-stroke RX-1 snowmobile, it seems as if the two-stroke engine is being left behind in the rush to meet exhaust emissions standards.
That is an illusion. Effective two-stroke emissions-control technologies not only exist, but are in production and being actively

developed. With these technologies, the two-stroke is transformed into a fuel-efficient, low-emissions power source. The question of whether to meet design goals with two-stroke or four-stroke has become entirely an economic one, answered by a given manufacturer’s needs (and capabilities).

Yamaha's Genesis Extreme 1000cc Four Stroke

Ski-Doo’s Semi Direct Injection (SDI)
800cc Two-Stroke

While Ski-Doo has shown a 1000-cc V-twin four-stroke snowmobile engine and a related three-cylinder four-stroke watercraft unit, they are also producing two-stroke sled engines that are expected to meet upcoming emissions levels through use of what the company calls Semi-Direct Injection (SDI). Yamaha’s success with four-stroke technologies has not stopped that company from continuing to produce and develop its HPDI (High Pressure Direct Injection) technology for its large two-stroke outboard engines.
In other words, two-stroke and four-stroke are parallel paths by which to reach low emissions levels.

All spark-ignition engines under emissions regulation face the same task of mixing fuel with air while not losing any fuel to the exhaust. They must then compress and burn the mixture of fuel and air efficiently so that almost no unburned or partially-burned fuel remains to pass out as exhaust. They must also burn the fuel-air mixture at a temperature low enough to avoid the creation of high levels of nitrogen oxides – the powerful pollutant that catalyzes formation of photochemical smog. At the same time, all of these engine types must control losses from friction and from the pumping effort required to fill and empty their cylinders. Fuel burned to cover such losses is a waste, and must be reduced to the lowest possible level.

It is easy to fall into thinking that all of the above goals are easiest met by four-stroke engines. US, Japanese, and European automakers had a brief romance with two-strokes in the period 1987-91. The development of direct fuel injection systems (DI) had removed the main drawback of the two-stroke – loss of fresh charge out the exhaust port before compression can begin. With charge loss stopped by DI, the advantages of the two-stroke stand out. They are;
(1) Low friction as a result of having no valve mechanism
(2) Low pumping loss from nearly equal pressures acting above and below its pistons (pumping loss is considerable in four-strokes operated at part-throttle).
(3) Simplicity - a low parts count (two-stroke cost-per-horsepower was estimated in 1991 as 70% of what an equivalent four-stroke would cost).
(4) Compactness. Low engine height results from having no valve mechanism above the cylinder head.
(5) Naturally low emissions of nitrogen oxides (NOx), the most difficult-to-control of pollutants. This results from the two-stroke’s “natural EGR”. The unavoidable dilution of the fresh charge by exhaust residuals reduces combustion temperature, thereby cutting NOx generation.
(6) A combustion chamber of such simplicity of form that it burns its fuel-air charge approximately twice as fast as an equivalent four-stroke, whose chamber is complicated by the presence of valves.

The automakers put four-stroke-trained engineers on their two-stroke programs. At the time of cancellation, automakers cited doubts as to how the engines would fare in extended durability, and uncertainty over future EPA regulations (tell us about it!). There was a bigger problem. Engineers lacking two-stroke experience spent a fortune “re-inventing the two-stroke wheel”. This persuaded management that two-strokes were a financial black hole - the end of the automotive two-stroke romance.

Re-evaluation of the two-stroke engine had come in the mid-1980s when Orbital Engine Company revealed its air-assisted direct fuel injection (DI). The central problem of the traditional two-stroke is its loss of fresh charge out the open exhaust port before compression can begin. Direct Injection solves this problem by injecting fuel directly into the combustion chamber, after the exhaust port has closed. Ordinary automotive injection could not accomplish this because there was too little time for such large fuel droplets (50-150-micron size range) to evaporate. Evaporation time is from exhaust port closure to the point of ignition, or about 70 crank degrees. At 6000 rpm this translates to about two thousandths of a second – not much time.

Orbital’s technology achieved a fast-evaporating 10-micron droplet size by injecting fuel into a pre-chamber, then blasting that fuel through a small orifice, into the main chamber, by admitting pressurized air into the pre-chamber through a small poppet valve. Experimentation has shown that a mist of 10-micron droplets burns as if it were fully evaporated, making the 10-micron droplet the “holy grail” of two-stroke DI.

2003 Ski-Doo Grand Touring SE

Traditional carbureted two-stroke engines begin to fire irregularly once the throttle is closed to about 30% load. As the throttle is closed, less charge is admitted to the cylinders, increasing the proportion of residual exhaust gas. Eventually the small charge is so diluted by exhaust that a spark cannot ignite it reliably, and misfires occur. At idle this situation becomes extreme, so that it can take several revolutions for a cylinder to accumulate enough charge to fire. This gives the carbureted two-stroke its characteristic irregular “ring-ding” idle and rough operation at light load.

DI eliminated this by creating a charge-rich zone near the spark plug by stratifying the charge – shaping the piston and head, and aiming the fuel plume so that most of the charge could be directed to where the spark plug could ignite it.

It is interesting to note that this same stratified-charge technique is now being applied to four-stroke automobile engines in order to operate them in the super-efficient “lean-burn” mode. The very same Direct Injection technology developed in the late 1980s for two-strokes is the basis for this new four-stroke low-emissions, low-fuel-consumption technology being currently adopted.

Two-stroke DI with low-load stratified charge gives a two-stroke engine the same steady, regular idle as a four-stroke. Because it cuts off all loss of fuel out the exhaust port, DI greatly increases fuel economy and cuts exhaust emissions of unburned hydrocarbons and carbon monoxide (CO). The result is a highly efficient and low-emitting engine. At one time, DI technology was regarded as complex and expensive, but the increased complexity of systems required on automotive four-strokes has made this seem routine and practical. All engines of the future – of whatever type - will carry heavy loads of sensors, actuators, and computing power to make them give the performance we want while preserving air quality and fuel resources.

A special problem that DI engines can have is making the transition from stratified-charge operation at low load, to homogeneous-charge operation (completely mixed) at mid- to full throttle. Yamaha, in their large off-shore two-stroke outboard engines, have therefore not adopted stratified-charge combustion for low load. Knowing that big outboards spend most of their time at mid-to-full throttle, they operate these HPDI (High Pressure DI) engines in homogeneous-charge combustion mode at all times. Low-throttle fuel consumption is a bit higher than with stratified-charge, but everything is a compromise. Meeting emissions and price levels are the big goals.
This bring us to Ski-Doo SDI – a quite different and more economical route to low two-stroke emissions and fuel consumption. SDI stands for Semi-Direct Injection, and it describes a system by which fuel is injected, not into the combustion chamber, but into the fast-moving air streams in the rear transfer ports. Because injection into the transfer streams more than doubles the amount of time available for evaporation of the fuel droplets, specialized fuel injectors like those of Orbital and Ficht are not needed. Direct Injection must wait for the exhaust port to close before injection can begin, but with SDI, keeping fuel from reaching the exhaust is accomplished by the natural scavenging flow pattern of a high-powered two-stroke.

In early two-stroke engines of the 1955-1980 period, the streams of fresh fuel-air mixture jetted into the cylinder from the transfer ports and mixed intimately with the residual exhaust there. This mixing inevitably carried much fresh charge out the exhaust, resulting in high fuel consumption and emissions. The process of improving engine scavenging has been a process of orchestrating several scavenge streams into a coherent, layered column of fresh charge, rising up the non-exhaust cylinder wall toward the head. In this column the highest velocity is on the wall, with lower velocities farther from the wall. There is some mixing between layers, but the important thing is that the rising mixture on the wall is effectively isolated from other flows that can reach the exhaust. In effect, the scavenge flow of modern two-stroke engines is naturally stratified. If fuel is injected into this wall-hugging transfer stream, that fuel is protected from loss for some time by the moving layers of air next to it, by the orderly nature of scavenge flow. If that fuel is injected early enough in the cycle, the leading edge of the fuel plume will reach the exhaust port and be lost. However, computer control of injection timing has allowed Rotax engineers to map the times when fuel can be injected into the transfer streams without loss, even though the exhaust port is still open as injection begins. This map information controls fuel injection timing on every SDI engine that is produced. Natural scavenge stratification also allows this system to operate more smoothly than a carbureted engine, right down to idle.

Yamaha have made some use of this same effect in their HPDI outboards, which do not switch to stratified-charge combustion at low load. Yet this same company dropped the bombshell of powering its top-of-the-line snowmobile with a 1000-cc four-stroke engine, derived from a sports motorcycle application. This is a bold move because it gambles the company’s market position on a brand-new strategy. Other snowmobile makers offer four-strokes, but mainly for low-performance “park” sleds and family fun units. This gamble, more than any other event, has persuaded many people that the two-stroke engine really is dead (or will be very shortly).
The facts are different. Yamaha’s reasons for putting a motorcycle four-stroke engine into a high-performance snowmobile are strongly influenced by economics. Japan has been in recession since 1997 and because about 1.25 trillion dollars of Japanese bank assets are tied up in non-performing loans, there is little money to lend manufacturers for new-model development. Yamaha have had to look to foreign capital markets, and to join the Japan-wide swing to shift parts production to Chinese plants. Money is therefore the key to everything Yamaha are doing. Development costs on the R1 motorcycle four-stroke are already spent. Adapting that unit for watercraft and snowmobiles was the best play they could make, meanwhile employing public relations to convert necessity into a “four-stroke revolution”. Honda have done the same, putting auto engines into outboards, and a motorcycle-derived engine into their Aquatrax personal watercraft.

Meanwhile Bombardier, responding to lesser but still real economic constraints, have had to decide what will power new snowmobiles and watercraft. This company is in a commanding position with respect to DI, not only having an Orbital license but having recently bought the US company Outboard Marine (OMC), who in turn own(ed) the Ficht DI technology. Both of these systems have a future, not only as two-stroke DI, but as noted above, in the coming lean-burn GDI four-stroke technology for auto engines.
SDI, according to Ski-Doo literature, achieves a fuel and oil consumption cut of up to 25% and emissions reduction by as much as 50%. Not only is this a clever, money-saving alternative to DI, it is “the next generation of Rotax 2-TEC snowmobile engines”, according to Jose Boisjoli, president of Bombardier’s recreation products division. I spoke to engineer Bruno Schumacher, who indicated that further research in injector position and timing is expected to increase the benefits already seen, and that there is no reason why this technology cannot have a long life ahead of it. Cylinder head and piston crown do not have to be redesigned to work with SDI (they do with bi-modal DI), and ordinary automotive-type fuel injectors, working on a normal 60-psi injection pressure make no call on high technology.

Now what about four-strokes? Are they the automatic and total answer many take them to be?
In fact, the designer of a four-stroke faces the same problem of keeping fresh charge away from the exhaust that the two-stroke engineer does. In school we learned that the four functions of intake, compression, power, and exhaust are completely separated in the four piston strokes. If this were literally true, fresh charge and exhaust gas would be always and perfectly separated. In actual practice, they cannot be. In order to have the intake valves fully open at the right time, they must begin to open before top center. Exhaust valves must likewise close slightly after top center in order to be sufficiently open during the exhaust stroke. This period around TDC after the exhaust stroke, when both intake and exhaust valves are open slightly, is called overlap. When the highest power is required, wave action in a correctly-dimensioned exhaust system is combined with increased overlap. This allows a negative wave in the exhaust to begin pulling fresh charge from the intake system, even before the piston has started to descend on its intake stroke. Torque is increased because this makes the intake cycle longer, improving cylinder filling.

If the engine is carbureted, the negative exhaust wave pulls air and fuel from the intake, and some is lost out the exhaust. This is acceptable in a racing engine, but in emissions-regulated engines it is not. Therefore Yamaha’s four-stroke snowmobile engine, which is at present carbureted, must limit overlap timing to keep emissions low, and must therefore forego this source of extra power. If that extra power is needed in future models, fuel injection must be adopted on this engine, as indeed may be required if emissions regulations tighten further. Four-stroke fuel injection timing can be controlled, just as it is in Ski-Doo’s two-stroke SDI system, so that fuel is injected only into air that cannot reach an open exhaust port.

Seen in this way, two-stroke and four-stroke engines are not as much different as they are similar. They are two different types of air pumps, but both must intake, compress, burn, and exhaust air and fuel, with the most complete combustion and without loss of fuel to the atmosphere. Both must display low losses in friction and pumping, both face the same economic laws governing their manufacture and sale. Both require considerable help from sensors and computers to accomplish their goals.

The two-stroke is light and compact. The four-stroke, while heavier, has an exhaust sound that many prefer to the two-stroke, and a powerband that is flat and easy to use. Both can meet emissions regulations, and both require computers and sensors to deliver their performance.

As Ski-Doo engineer Bruno Schumacher said to me, “Snowmobiling is the ideal two-stroke kingdom, because the sled must float on snow.” On the other hand, test riders loved the Yamaha on groomed trails. Horses for courses.
Schumacher concluded, “There is room for both. There will be parallel roads”.