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112 comments on An Ethanol Bright Spot

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Black_Dog on January 24, 2009 - 3:55pm Permalink | Subthread | Parent | Comments top

The efficiency of an IC engine is directly related to the compression ratio (CR). Actually, as I recall from my IC engines class back 40 years ago, the efficiency was actually related to the expansion ratio, which is usually the same as the mechanical compression ratio (i.e., that which happens at wide open throttle). Since gasoline has a problem with detonation or pre-ignition, lower mechanical compression ratios have been traditionally used compared with diesel designs.

There are other engine designs which have been tried, such as the Miller Cycle in the Mazda Millennium S about 10 years ago. That design used a mechanical supercharger, as I recall. Turbo chargers also produce a similar result, as they run lower mechanical CR's but "recycle" some of the energy from the expansion of the exhaust gases to increase the effective compression ratio and also increase the effective displacement of the engine. Some designs have just used a turbine attached to the output to directly capture some of the waste energy from the exhaust. Then, there's the variable valve timing and knock sensor options, which can make it possible to use higher mechanical CR's while running lower octane gasoline.

With gasoline (or alcohol) as a fuel, achieving the best efficiency is accomplished by running the motor at wide open throttle. This fact has led to ever better transmissions, such as the continuously variable designs or the 6 speed automatics, which make it possible to keep the engine running at nearly full throttle under most situations. The smallest engine usually gives the best MPG, since it will be fun nearly flat out all the time. In my view, the Toyota Prius design functions like a CVT with energy storage.

E. Swanson

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ThatsItImout on January 24, 2009 - 6:22pm Permalink | Subthread | Parent | Comments top

E. Swanson,

You hit the nail on the head, it is "effective compression ratio" that matters. This can be accomplished by all manner of tricks, from extremely advancing the timing and lean burn designs to the use of turbocharging/supercharging.

Much good work was done on this in the 1970's in relation to using propane as a motor fuel. As we know propane and compressed natural gas suffer from a BTU penelty compared to gasoline, but in an engine designed to use it, the power output and fuel mileage can be roughly equal. High performance engine builders in the 1970's such as Ak Miller and Gale Banks even used propane fueled engines for racing and high performance boats.

What the designers of those days lacked were the computer controlled fuel injection systems we have now. The way to preserve fuel flexibility so that gasoline and E85 can be used in the same engine with roughly comparable efficiency is through the use of advanced variable vane turbochargers and advanced fuel and spark control. As Robert points out, we cannot raise the mechanical (as opposed to "effective) compression ratio beyond a certain point or we lose the fuel flexibility.

As an aside, I was just watching the start of the Daytona 24 Hours sportscar race on TV. Efficiency of the engines in these cars can be fantastic, sometimes almost 200 horsepower per liter with turbocharging. The efficiency in the case of racing is used for performance and speed, not fuel efficiency (although there is a fuel consumption limit). This has been true on the highway as well, in which fuel conversion efficiency of modern engines is much better than a few years ago but we have used it to gain horsepower, speed and weight of the vehicle, not for fuel efficiency.

RC

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112 comments on An Ethanol Bright Spot

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