New Possibilities

New possibilities

TDC

Kevin Cameron

ONCE UPON A TIME THERE WERE JUST two kinds of engines-the spark-ignition type in cars, light trucks and motorcycles, and the compression-ignition (diesel) in heavy trucks and industrial machines. Forget that. Variety is the word today.

For example, the new Cadillac CTS V-Six is a variant called a gasoline direct injection (GDI) engine. Only pure air is drawn into its cylinders and the fuel is injected directly into the combustion chambers later. Without evaporating fuel taking up space as vapor in the manifold and intake ports, more air is drawn into the engine, increasing volumetric efficiency. Because the injection can create a rich, ignitable zone while the overall mixture is very lean, there is potential for serious reduction in fuel consumption. When I asked Marelli engineer Stefano Perotta if GDI is being studied for possible use in MotoGP, he replied that it had already been studied (and banned) in Formula One. Hmmm, so there are paths to improved fuel consumption in MotoGP.

Turbochargers have been adopted on small engines because they allow us to combine spirited performance with the superior around-town economy of a dinky engine. But.. .you have to wait for the turbo to spool up. Ricardo Consulting Engineers in England is working on a dualmode engine with a four-valve head that can, on demand, switch from two-stroke to four-stroke operation. Here we are at low revs and we suddenly need to accelerate, so two-stroke mode kicks in (our valves are not operated by cams but by other, more versatile means) and we have the twostroke’s high torque to thrust us forward. As our engine revs up and there is no longer time enough to fill its cylinders in two-stroke mode, it switches to four-stroke and carries on to high revolutions.

Another type of multi-mode engine now under development is related to Honda’s EXP-2 “sparkless” off-road bike of the mid-1990s. It used a variable exhaust gate to enable just enough hot gas from the previous cycle to be trapped in the combustion chamber; as the fuel mixture was further heated by compression, it would spontaneously ignite in myriad places. This kind of combustion is now called “HCCI” for Homogeneous Charge Compression Ignition. This is not a diesel-in a diesel, pure air is compressed to so high a degree that an injected fuel spray ignites shortly after contact with it. But in HCCI, fuel is mixed with charge air before compression. The advantage of HCCI is that it can run at very lean fuelair ratios with low flame temperature, never making heat sufficient to generate hard-to-eliminate nitrogen oxides. Thus, an HCCI engine might achieve diesellike fuel economy without the expense of a diesel’s structural strength (necessary to withstand compression ratios of 17 or more to 1), complex injection system (estimated to add 30 percent to engine cost) and advanced emissions controls.

HCCI’s special problem is that at idle or low loads there isn’t enough exhaust heat available to keep combustion going, while at full throttle filling the cylinder with fuel-air mixture leaves no room for ignition-enabling hot exhaust gas to be included. Therefore in these two conditions, the engine must switch back to conventional spark ignition and nonlean-burn. Switching smoothly between spark and HCCI modes presents special problems. This research is being vigorously pursued because automakers, soon having by law to achieve Corporate Average Fuel Economy (CAFE) of 35 miles per gallon, hope there is a cheaper way to do this than turbo-diesel power or hybrid powerplants.

In extreme fuel-economy contests, pumpkin-seed-shaped low-drag vehicles are driven by tiny engines that operate intermittently and always at full throttle. When the engine is stopped, the vehicle

coasts. In this way, mileage as high as 1200 miles per gallon is achieved. Naturally, acceleration is unexciting.

Why not operate the engine all the time but on partial throttle? The answer is friction and pumping loss. Obviously, stopping the engine part of the time reduces the fuel used to shear engine oil films. Plain bearings are maximally efficient at their maximum capacity-at loads just short of oil-film failure. It is therefore more efficient to size them for such operation at full load, and avoid any operation at lower loads.

Not so obvious is pumping loss-negative work performed in pulling a partial vacuum in the cylinder during the intake stroke at part-throttle. This pumping loss increases as a four-stroke engine is throttled but is nearly eliminated by operation at full throttle. Therefore for extreme economy, the engine is operated intermittently, always at füll throttle.

Is there a more practical way to obtain these advantages? There is: hybrid power. Instead of operating the internalcombustion (IC) engine inefficiently at part-throttle, low-load operation is powered from a separate battery/electric-motor drive system that is later recharged when the IC engine can be operated at closer to wide-open throttle. Because hybrids have two powerplants, there is a cost penalty. Balance first cost differences against comparative fuel costs during your ownership and draw your own conclusions as to whether hybrid or conventional drive is cheaper overall. Now repeat the calculation at $8 a gallon, just for fun. This is our world.

The above also partly explains the former interest of automakers in fuel-injected, crankcase-scavenged two-stroke engines around 1990. When such engines are throttled, pressure drops both above and below the pistons, thus essentially eliminating part-throttle pumping loss. Uncertainty as to what the EPA might want next brought most of this research to an end.

Not long ago, proposals to build multicombustion-mode engines or engines whose entire pattern of valve operation could radically change in a split-second would have been laughed out of the boardroom as Buck Rogers nonsense. But with that 35-mpg law staring engineers in the face, all kinds of radical ideas look not only possible but downright attractive. □