No Perfect Answers

Is there an ideal snowmobile engine? Is there a perfect combination of low emissions, light weight, high power, and reliability? With each new release of technology there have been announcements that this is really ‘it’ – the engine to believe in, the powerplant of our dreams.

The first ‘answer’ was electronic fuel injection – in its throttle-body form, keeping fuel mixture correct and economical despite changes in barometer, altitude, or temperature. Then came two-stroke direct fuel injection (DFI) – cutting unburned hydrocarbon emissions (UHC) by 75% or more. Even as DFI engines were reaching the market, opinion seemed to jump to four-stroke engines, which could reach low emissions levels without needing complex injection systems. When DFI two-strokes and the new four-strokes reached the user, both achieved good fuel economy and low emissions. People loved the four-stroke sound but the associated extra weight was like forever carrying a passenger. Now, to boost power-to-weight ratio, smaller four-strokes with turbo- or other boost are proposed as the next revolution. Can the industry make such reliable as well as powerful?

Arctic Cat Turbo 660 4-Stroke

What will EPA want next? Which existing technologies will thereby be swept away, and what others put in their place? The answer is that sled engine design is a moving target, and as one 19th-century general noted, “No plan survives contact with the enemy”. Our industry is having to generate the answers as it goes, and to understand what they mean, we have to find the facts and examine them critically. Just remember that a few years ago, the gasoline additive MTBE was hailed as savior of the atmosphere and multi-million-dollar plants were built to produce it. Shortly later MTBE became illegal because its solubility in groundwater had polluted thousands of wells. We humans are clever, but our truths are not absolute – we approach truth by trial and error. Advertising’s job is to simplify complicated choices, but the consumer must read past Page One to discover the real issues.

Arctic Cat 900 EFI 2-Stroke

Initially, conventional carbureted two-stroke engines passed through intensive cylinder and pipe development which over 30 years has increased their power and cut their fuel consumption. Such engines can now give over 200-hp per liter of displacement, while peak-power fuel consumption has fallen from the vicinity of 0.85 lb/hp-hr to 0.65 – a drop of 20-25%. These are extremely light powerplants, weighing typically less than one pound per horsepower.

EFI
An early response to emissions standards was testing and limited production of electronic throttle-body fuel injection systems (ETBI or EFI). In this system, throttle butterflies and fuel injection nozzles are located where carburetors used to be. The great potential advantage of electronic TBI is that fuel delivery is continually adjusted by electronics and sensors to maintain exactly the desired fuel/air ratio. In a carbureted production engine, carburetion must be set rich enough for safe operation in the coldest foreseeable weather. This, of course, makes the fuel mixture richer in warmer weather, or at higher altitudes – causing increased UHC emissions. Mechanical and electronic float bowl pressure regulators have since proven to be effective partial solutions to changing environmental conditions, providing cleaner operation with a better match of fuel flow to the current conditions.

In racing, carburetor jets and needle positions are changed as necessary to compensate for changes in barometric pressure and temperature. Without such attention production carbureted sleds run rich in warm weather or at higher altitudes, and leaner in cold weather, lower altitudes, and higher barometric pressure.

Electronic TBI is therefore like having an expert racing tuner constantly adjusting the carburetion as conditions change. While this both increases power and reduces emissions, it does not attack the underlying emissions and fuel consumption problem of the two-stroke engine – that it clears and refills its cylinders not with pure air as Diesel engines do, but with carbureted fuel-air mixture. Because transfer and exhaust ports are open together during the cylinder refilling process, in carbureted or TBI engines some fuel is inevitably lost out the open exhaust port to become UHC.

DFI ARRIVES
In response to this, Direct Fuel Injection (DFI) systems were developed. These eliminate the loss of fuel through open exhaust ports by injecting fuel directly into the combustion chamber only after the exhaust port has closed. Because there is little time between exhaust closure and ignition, this requires use of special high-speed, fine-particle-size injection equipment, quite different from the automotive electromagnetic fuel injectors on the engine of your car or truck. The two familiar DFI systems are the Orbital and the Ficht.
Both of these systems reduce emissions of UHC by something like 75-80%, and improve fuel consumption down to or below that of four-stroke engines. While a sharply-jetted carburetor or TBI two-stroke might need 0.65 lb/hp-hr of fuel, the new breed of DFI engines deliver economy in the 0.45-0.5 range.

DFI PROBLEMS
There has been controversy surrounding DFI systems as applied to outboard motors or small automotive engines. Certain early Ficht-equipped outboards suffered plug fouling if operated for hours at a time near the transition between those engines’ two modes of combustion. At lower throttle, greater economy is obtained by operating in stratified-charge mode (fuel spray is rich enough to be spark-ignited, but is too lean elsewhere), while at higher throttle, the entire charge contains fuel (homogeneous charge mode). Ficht engines in other applications have been well received. In the automotive world, early interest in DFI two-strokes faded over EPA regulatory uncertainties, but large two-stroke vehicle fleets have been successfully operated in Europe and Australia.

Polaris 1200cc Direct Injection 2-Stroke

SEMI-DIRECT INJECTION
Add to this picture the Bombardier-Rotax creation of Semi-Direct Injection (SDI), which employs low-cost hardware to achieve most of what DFI delivers. SDI has been used successfully for several years on Sea-Doo watercraft and appears suitable for snowmobile engines as well. The main reason that SDI works as well as it does is that development of two-stroke scavenging flows has achieved good separation of fresh charge and exhaust flows – essentially stratifying the air motion in the cylinder. By injecting fuel selectively into a part of this flow, far upstream in two of the transfer ports, fuel can be kept from reaching the exhaust.

While the SDI systems offered on 2004 Ski-Doo models are quite advanced compared to the first Sea-Doo injection systems, Bombardier SDI is still at a fairly early stage in its overall development. Company engineers are optimistic that further gains are possible within its simple concept. While full DFI can add 20-30% to the cost of an engine (this is true of Diesels as well), SDI might be achieved at cost similar to a carburetor system.

How about adding catalytic converters to our current fleet? Carbureted or TBI two-strokes lose too much fuel to their exhaust for the application of catalytic mufflers. The more UHC the catalyst must burn, the hotter it operates. DFI and SDI engines, with already little UHC in their exhaust, can be further refined by catalyst use if standards tighten again.

Rotax 2-TEC 600 HO SDI

Right now, four-stroke engines can (and do) meet sled emissions numbers even with carburetors, but reaching higher levels will require the greater accuracy and self-tuning features of electronic fuel injection. Further, a four-stroke’s combustion has to be more intense than that in a two-stroke in order to make equal power while firing half as often. This higher temperature combustion naturally produces more oxides of nitrogen (NOx), while a two-stroke’s natural EGR (inevitable recirculation of exhaust gas from one combustion cycle to the next) lowers combustion temperature, making the two-stroke naturally low in NOx. Any change which raises an engine’s combustion temperature (higher compression ratio, turbocharging, &c) also raises NOx production.

FOUR-STROKE ADVANTAGES
Four-stroke engines have solid advantages. Because of good separation of the four functions of the engine cycle – intake, compression, power, and exhaust – it is easy to keep fresh charge from being lost into the exhaust system to become UHC. Provided that moderate valve overlap is used, this can be achieved even with carburetors – as seen on Yamaha’s RX-1. Lubricating oil is segregated from the gas exchange process and the crankshaft and other parts are lubed by circulating oil, allowing long parts life. Effective piston oil scraper rings keep oil from reaching combustion chambers. Because most of a moderate performance four-stroke’s gas exchange process depends upon piston pumping, four strokes have very wide torque curves, helping to broaden their consumer acceptance.

Yamaha Genesis Extreme 1000cc 4-Stroke

AND DRAWBACKS
Four-strokes pay for these advantages by being about 40% heavier and bulkier than a two-stroke of equal power, as those who bought Yamaha’s 140-hp RX-1 discovered. Although charmed with the power and muscular sound, reviewers cautiously noted that the machine was ‘best on groomed trails’, as its weight complicated operation on uncompacted surfaces.

Where is the extra weight? The obvious place to look is the valve train – valves, tappets, cams, cam drive(s) and their lubrication arrangements. The less obvious place is in the extra engine stiffness required to let a four-stroke rev high enough to compensate for firing only every other time each piston comes near TDC. When you spin a crankshaft faster, the loads from whirling counterweights and reciprocating pistons and rods increase as the square of the rpm. Reliably supporting those increased forces requires extra metal, tying cylinders and main bearing saddles together. Matching the power of a two-stroke with a four-stroke requires either increasing displacement or rpm - or both. To remain at a reasonable piston speed at higher revs usually means increasing the number of cylinders to obtain the necessary short stroke. All of these mean extra weight.

POWERING UP
Simplicity is lost when more power is needed. To flow adequate air at higher crank speed, valve timings must be extended – including the so-called “overlap” – the period at the end of the exhaust stroke when the intake valves have begun to open but the exhaust valves have not quite closed. Now the four-stroke engine begins to act more like a two-stroke, because charge motion during valve overlap is driven not by the piston (which is sitting almost still near TDC) but by wave action in the exhaust pipe. Ideally, an exhaust suction wave from a correctly-dimensioned header pipe arrives back at the cylinder during overlap. It pulls residual exhaust out of the cylinder, causing fresh charge to be drawn in from the rapidly opening intake valves. In effect, this gives the intake stroke a head start, and by sweeping out the residual exhaust gas above the piston at TDC, it prevents ill effects caused by dilution of the fresh charge, such as poor idle stability. Now the bad news; as you increase valve overlap to get more power, you begin to lose fresh charge into the exhaust - just as happens in carbureted two-strokes. To avoid this, the four-stroke engine now needs electronic fuel injection so the fuel spray can be delayed until after the exhaust valves have closed.

Just as happens with a two-stroke, at some lower rpm the tuning effect of exhaust waves is reversed and acts to reduce torque. Now a positive exhaust wave is reflected by the pipe, back to the valves, where it pushes exhaust gas into the cylinder and even back through the intake pipe into the airbox. This produces a torque flat spot, typically centered on 70% of peak-power rpm.

Polaris 750cc 2-cylinder Turbocharged 4-stroke

MAKING LIGHTER FOUR-STROKES
One response to four-stroke weight is to increase the power of a smaller, lighter engine by some form of forced induction (turbocharging or supercharging). An engine’s power is roughly proportional to its manifold pressure, so if you double the full-throttle manifold pressure with a blower, you will roughly double the engine’s power. The only weight increase will be that associated with the induction system – of the order of fifteen pounds. Polaris’s 750 twin (PWC) and Cat’s 660 triple turbo are projects of this kind.

The success of this method depends upon making the engine reliable under substantial intake boost, as the higher boost is pushed, the more the engine comes to resemble a Formula One engine of the 1980s. Several potential problems must be solved. First, to avoid detonation, the engine’s compression ratio must be reduced. This increases part-throttle fuel consumption. Even so, piston cooling may have to be improved by directing oil jets at the undersides of their domes, requiring a bigger oil pump and perhaps improved oil control. Second, because boost increases peak combustion pressure, cylinder head gasket and hold-down studs must provide improved sealing and higher clamp load. To avoid gasket problems, some racing four-stroke engines are now being built with liners that screw into the cylinder head. A third class of problem centers on heat. While exhaust valve problems are rare in conventional liquid-cooled four-strokes, when such engines are operated at high boost, exhaust valves may have to be made of exotic nickel alloys and valve seats and stems may require what the industry calls ‘strategic cooling’. This means the provision of high-speed coolant jets directed at the hottest parts in the cooling circuit.

EMISSIONS TRENDS
If the EPA continues as it always has, emissions limits will tighten. This suggests snowmobile levels will follow the path taken by auto emissions. For four-strokes this may mean fuel injection, catalytic mufflers, air pumps, and possibly, in time, lean-burn combustion. Curiously, the basis for automotive lean-burn, stratified-charge technology is the very same type of high-speed, fine particle size fuel injector developed for two-stroke DFI!

If NOx levels are lowered, it will be harder for four-strokes to meet them than it is for two-strokes, as noted above. That might one day require the full automotive three-way exhaust catalyst system for four-strokes.

DIESELS IN THE FUTURE?
Automotive engineers are closely studying what is happening in Europe, where 40% of new cars are sold with small, highly efficient common-rail turbo-Diesel engines. A change in emissions legislation would be required to do the same in the US, but pressures are building because Diesel efficiency is 30-40% better than that of gasoline engines. Such engines are, thanks to their improved fuel injection systems, hard to tell from gas engines in their operation. For these reasons we have to consider them as a possible future snowmobile choice as well.

All of the above makes it difficult for engineers planning the future of snowmobiling. There are no easy, automatic answers. This is surely why Bombardier are consciously pursuing a ‘dual-path’ strategy with both two-stroke and four-stroke engine development. Since that company bought out Outboard Marine (makers of Evinrude-Johnson outboard engines) they have gained control of OMC’s rights to the Ficht DFI technology as well. Earlier Ficht solenoid-driven DFI injectors now in use on watercraft engines have now been supplemented by new voice-coil-driven injectors whose higher rpm capability may qualify this system for snowmobiles as well. Yet on the other hand indications are Yamaha may be quietly pursuing an all-four-stroke future. In just one short year, the Yamaha RX-1 has swayed the industry to the point where nearly 25% indicate they’re more interested in four-stroke technology for future purchases! We await Yamaha’s next designs with interest.

Engineers live with these fascinating complexities every day. Advertisers try to simplify the choices for us with appealing slogans. We the consumers must try to unravel the whole tangled business as best we can.