why do some smaller engines have lower gas mileage than some larger engines?
In: 208
It takes energy to cause a piston to move, and it stops at each end of each stroke. Every time you move that piston, you expend energy just driving it. The faster you move it, the more energy you waste per stroke. The more frequently you move it, the more energy you waste. This is (part of) “pumping loss”. Broadly speaking, this is the energy the engine consumes within itself in order to be able to produce a useful output.
To develop a net 200HP at the output shaft, a small engine might have to turn 6000 RPM. The total power produced might be closer to 300HP, but 100 of that is wasted just moving the engine’s internal components. This engine, at this 200HP output, will be extremely inefficient.
A larger engine might produce 200HP out at 900RPM, with just 10HP in pumping losses. This engine, at this output, will be very efficient.
If you only need 10HP, though, that second engine might need to turn at 600RPM, with 9HP lost to pumping. The first engine might be able to achieve it at 900RPM, with 1HP lost to pumping. At this lower output, the smaller engine will be much more efficient than the larger.
The key to efficiency is matching your engine’s output to your load; or your load to the engine’s output. The greater the mismatch between optimal output and actual load, the less efficiently the engine will operate.
Latest Answers