Why doesn't increased compression ratio increase feul economy?
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From: Traveling, tn
Why doesn't increased compression ratio increase feul economy?
If you take the same amount of fuel and compress it more then you get more power. So it seems to me that an engine with a high compression ratio should be able to produce the same amount of power with less fuel than a lower compresion ratio engine. However in reality it doesn't seem to work that way. Where is the error in my logic?
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Re: Why doesn't increased compression ratio increase feul economy? (fredtoast)
It's not that simple. High-compression engines are generally designed for power, not economy.
In fact, at low-load operation (say cruising at constant 45mph) you only need like 10 hp, so the best thing would be a 50cc engine geared to run at 12,000 rpm & wide-open throttle. But then it would be unable to provide 100 hp when you need it.
What we need is an engine that always runs at WOT, but it magically grows & shrinks in displacement.
In fact, at low-load operation (say cruising at constant 45mph) you only need like 10 hp, so the best thing would be a 50cc engine geared to run at 12,000 rpm & wide-open throttle. But then it would be unable to provide 100 hp when you need it.
What we need is an engine that always runs at WOT, but it magically grows & shrinks in displacement.
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From: Charlotte, NC
Re: Why doesn't increased compression ratio increase feul economy? (JimBlake)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JimBlake »</TD></TR><TR><TD CLASS="quote">What we need is an engine that always runs at WOT, but it magically grows & shrinks in displacement.</TD></TR></TABLE>
Not quite the same, but a little like a hybrid with a CVT. Put the engine at just the RPM required to make the requested torque, and leave it there. Supplement the smaller displacement with a little electric omph, and away we go.
I drove my uncles Prius the other day, that is a well done car. When I first stepped on the gas and heard it go wrrrrrrrrrrrrrr, I got nervous though. I have driven the Honda CVTs a lot, but they pretend to make little gear shifts. This one just revved and stayed there, which is the exact same sound an Acura 3.2TL with a burnt up transmission makes just before it leaves you on the side of the road.
Takes a little getting used to, but the car drives great, although a stock HF will out-mileage it. Not quite apples to apples though, the Toyota is pretty roomy.
Not quite the same, but a little like a hybrid with a CVT. Put the engine at just the RPM required to make the requested torque, and leave it there. Supplement the smaller displacement with a little electric omph, and away we go.
I drove my uncles Prius the other day, that is a well done car. When I first stepped on the gas and heard it go wrrrrrrrrrrrrrr, I got nervous though. I have driven the Honda CVTs a lot, but they pretend to make little gear shifts. This one just revved and stayed there, which is the exact same sound an Acura 3.2TL with a burnt up transmission makes just before it leaves you on the side of the road.
Takes a little getting used to, but the car drives great, although a stock HF will out-mileage it. Not quite apples to apples though, the Toyota is pretty roomy.
Joined: Jul 2004
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From: anderson, in, usa
Re: Why doesn't increased compression ratio increase feul economy? (fredtoast)
higher compression without compensation usually means a higher risk of detonation. what this means is since the compression of the motor is higher
it will need a richer (or more volatile) fuel mix to avoid detonation. wonder why you're supposed to
use premium fuel in most hondas, cuz of the compression ratio being higher. just because the fuel mix is compressed more doesnt mean there needs to be less fuel mix to do the same job. and the more compression you have the more difficult
it is to achieve full combustion. which is usually why higher compression motors use more fuel, and sometimes need bigger spark (ignition) to work right.
it will need a richer (or more volatile) fuel mix to avoid detonation. wonder why you're supposed to
use premium fuel in most hondas, cuz of the compression ratio being higher. just because the fuel mix is compressed more doesnt mean there needs to be less fuel mix to do the same job. and the more compression you have the more difficult
it is to achieve full combustion. which is usually why higher compression motors use more fuel, and sometimes need bigger spark (ignition) to work right.
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