Quote:
Originally Posted by drmiller100
First, lets define efficiency. For my purposes I define efficiency as making the most miles per gallon of fuel.
Which is more efficient - a 500 cc engine running at 4000 rpm or a 2 liter engine running at 1000 rpm)? If both vehicles have the same intake vacuum (measure of throttle and engine efficiency), then I would bet on the 2 liter.
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Ordinarily, the efficiency quoted for a given engine is based on energy out / energy in (which is derived from BSFC). Miles per gallon is usually not used as a measure of engine efficiency because it has as much or more to do with the car (and car/engine fit) as it does with the engine.
So, we have to make some assumptions. You mention "both vehicles", suggesting that the vehicles might be different. In that case there is no way to answer the question. Let's assume that it's the same vehicle with an engine swap, and that we have ballasted the car to the same weight in both cases.
We'll also assume that the engines are of the same general specific output class: say 60 bhp per liter.
In that case the 500 cc engine will be at half throttle and the 2000 cc engine will be at 1/8 throttle. RPM will not have a great effect on BSFC
For simplicity, we'll assume a single test speed, at which 11 hp is required (a small car cruising at 60 mph).
The two engines could be covered by the same BSFC map if one was a single cylinder version of the other (and had all other details the same).
Here's a BSFC map for a Saturn, a pretty typical engine:
For each engine, you'd want to pick the engine speed that gets you closest to the sweet spot. The torque figures along the right of the chart would be divided by 4 for the 500cc engine. For the 500cc engine I'll guess at 2500 rpm, which would require 23 lb-ft torque (31 nm) (which will show up as 124 on the chart). So BSFC would be about 230 gr/kWh.
For the 2000cc engine, at 1000 rpm 78 nm would be required, so consumption would be about 290 gr/kWh.
So the larger engine would consume 290/230 or 1.26 times as much fuel.
At lower cruise speeds, the difference would be much greater.