In petrol engine turbocharger applications, boost pressure is limited to keep the entire engine system, including the turbocharger, inside its thermal and mechanical design operating range . Over-boosting an engine frequently causes damage to the engine in a variety of ways including pre-ignition, overheating, and over-stressing the engine's internal hardware. For example, to avoid engine knocking (also known as detonation) and the related physical damage to the engine, the intake manifold pressure must not get too high, thus the pressure at the intake manifold of the engine must be controlled by some means. Opening the wastegate allows the excess energy destined for the turbine to bypass it and pass directly to the exhaust pipe, thus reducing boost pressure. The wastegate can be either controlled manually (frequently seen in aircraft) or by an actuator (in automotive applications, it is often controlled by the engine control unit ).
We’ll compare to an average oil-fired power plant that also uses liquid petrocarbons. I’ve got a 37% efficiency figure there. I believe distribution losses on the US grid are around 5%. So per gallon of gasoline, we get 42MJ/gallon. There is some difference in fuel types, but I think it’s negligible for these purposes. Let’s compare to a Tesla – which is arguably a bit unfair, as a Tesla grossly outperforms the equivalent 32 mpg ICE car, though it does have similar range. We’ll look at the Model S *85* series, which has an 85kWh battery pack (306MJ). This consumes gallons of oil at the power plant to charge. The EPA 5-cycle range (which results in worse numbers than the 2-cycle test above, in part by turning on heaters and air conditioning) results in a range of 253 miles on ludicrous mode. This puts you at 35 mpg, still better than an everyday ICE mpg, for really the worst case comparison – a Tesla on ludicrous mode, using only one of the least efficient fuels that constitute a small portion of the grid, compared to an everyday sedan.