The torque curve of a car engine indicates that the torque initially increases with engine rpm and after attaining a peak it starts to decline . How is it possible to run at higher rpm at lower torque since the increased quantity of fuel consumed at high rpm would result in increased torque output?
Once the torque reaches the peak, the volumetric efficiency of the engine reduces because the engine cannot breathe faster. The air intake valves are fixed and are based on timing. Hence the engine is tuned for maximum air intake around that rpm range. Once it crosses the optimum rpm range, the air intake reduces.
This happens even in electric motors, but is caused due to electromagnetic effects. Engines are designed for their intended use. The usual engines on the road are designed to yield fairly high torque values at low RPM, because this makes them easy to ride in day-to-day life, and Harley-Davidsons have their torque concentrated even lower in the RPM range than BMWs do.
Low-end torque is accomplished by several design traits, one being small valves and intake tubes which create high air velocity into the cylinder for good fuel mix at low speed.Those effects tend to become a restriction at high RPM, which means that engines intended for high RPM end up with larger valves, larger air intakes, smaller cylinders and other things that let them continue to breathe when other engines start to gasp.
There are technologies in cars, like variable valve timing and variable intake tract length which can give motors the best of both worlds by increasing torque at higher RPM without giving it up at low RPM. Incidentally, Honda has variable valve timing on a motorcycle now.But basically, the engines gasps for air at high rpms and hence the torque decreases. There are several ways of addressing this issue. At Hyundai it has been addressed it to a large extent in the company’s kappa and gamma engines, with the use of continuous variable valve timing (CVVT), wherein the timing of a valve lift is altered, thereby improving performance, fuel economy or emissions through use of electro-hydraulic systems. This helps in breaking the relationship to some extent.