A petrol engine is an air pump that is regulated by a throttle, and by its very name, the throttle does just that – it throttles how much air gets into the engine. However, in doing so, it creates a vacuum and this vacuum wastes a lot of energy. Your typical vacuum cleaner at home can easily consume 2kw so you can easily imagine how much wasted energy and fuel can go into creating inlet vacuum on a typical engine inlet manifold!
A diesel engine however is different, it has no throttle and hence creates no vacuum. So how do you control the power of the engine, I hear you say? Great Question! Diesel engines vary the amount of diesel that is directly injected into the compression chamber to increase or decrease power. Hence, no power is lost due to the throttle pumping losses
Interestingly, the CITS Engine is the only petrol engine that doesn’t require a throttle and has the same fuel efficiency advantages