Why does the efficiency of the gas engines drop so much when throttled down?
On the intake stroke the piston sucks the air/gas mixture into the cylinder. This takes energy from the piston. To slow the engine down there is a throttle plate that makes the piston work against a lower intake manifold pressure. If you only need 15 HP out of that 300 HP engine you could easily be using 45 HP creating the vacuum in the intake manifold; thus the engine is having to put out about 60 HP plus whatever is being lost to friction in the cylinder walls. Cadillac and a few others have tried just leaving the exhaust valve opened on some of the cylinders, so the working cylinders have a heaver load and hence working against less vacuum; but this does nothing to reduce the friction loss of the pistons against the cylinder walls. Incidentally, a diesel engine also gains because it does not have a throttle plate, it simply injects less fuel. But, this introduces the complication of injection pumps, which is certainly justified for larger engines. With newer techniques in the valve
