Quote:
Originally Posted by racprops
From all I have read about MPG I come up with two ways SMALL and LIGHT with small motor, to make them work they run higher RPM.
But I am driving a BIG van, big in wind resistance (pushing a box though the air) and heavy.
The trick seems to be a big TORQUE motor. There is a reason diesels are used on big rigs, TORQUE.
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The whole "torque is what gets you down the road" is something I take issue with, at least to some degree.
Let's say you have two engines, a 200 putting out 200 ft-lbs of torque and maxing out at 6000rpm, and a 400 putting out 400ft-lbs but with a (for the sake of argument) very low redline of 3000rpm. Both peak a little over 200 horsepower, but the larger engine does so at half the RPM.
If you put the 200 on a 2:1 reduction gear, torque to the wheels will be effectively
identical to the 400ci engine. It will spin two times for every one time the larger engine spins, firing twice, and having twice the mechanical advantage and producing the same torque and horsepower at the wheels.
The smaller engine will have more friction from spinning faster (this goes up exponentially, not linearly). The larger engine will have more friction from physically larger and heavier parts with more load placed on them. Neither one will be inherently more or less efficient. Both can be equally drivable.
What's going to improve efficiency are two things: 1) Keeping the engine as highly loaded as possible while driving, which means having a very wide selection of gears with a very tall final, and 2) having an engine design that is very efficient, big or small.
More RPM means lower economy, because friction goes up dramatically with rpm. But, with the same gearing, more torque also means lower economy, because engine load drops. So you want both small displacement AND low rpm.
All engines have an amount of power they make most efficiently. That amount will be larger on larger engines. While you can affect them to some (small) degree by changing things like cams, there are a lot of factors you can't as easily change such as bore to stroke ratio, piston velocity, displacement, sidewall loading, etc. Ultimately you want to match the engine to your vehicle whose most efficient power output is closest to the most typical power output you need, and there's almost certainly a stock engine somewhere which does what you need.
A great example to enumerate this would be a Ferrari following a Prius around a race track. With the Prius engine running at its very limits, the Ferrari will probably deliver better economy. Change the driving conditions though, such as to typical commuting, and the tables turn.
Honda's goal with the Insight was the smallest, slowest spinning engine possible - in this case 1L / 60ci. There are a handful of people on Insight Central who can reliably squeeze 100-140mpg out of these cars with careful driving and high tire pressure. There are far more people who average only 50's, just due to different driving patterns.
I guess if I can summarize my thoughts, it would be that you can get what you're looking for out of either a small or large engine, but in practice it's easier from a small(er) engine, so long as you don't go too small.
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