Auto Manifesto

February 8, 2008

Reshaping Drive Cycles

Every vehicle goes through a number of states during each use. It accelerates, cruises at a constant speed, decelerates, and stops. This even occurs when cruising, though the magnitude of the changes is smaller. Below is an example illustration.

Since most (all?) highway vehicle engines were, until recently, directly coupled to the drivetrain and wheels, there was no way to decouple the speed of the engine from the speed of the vehicle. The engine had to turn a certain rpm and the gears had to be a certain ratio to power the vehicle at a particular speed.

Not anymore. Hybrid powertrains have enabled engineers to smooth drive cycles out by recapturing energy during braking and using it to enhance acceleration. Start/stop engine technologies have also enabled automatic engine cutoff when the vehicle comes to a stop, further conserving fuel.

Instead of using powerful but thirsty engines to accelerate quickly and to reach high terminal velocities (or in some cases electronically limiting them) engines are being scaled down not just because they’re now more efficient than before, but also because less outright power is necessary when these engines are coupled with hybrid systems that enable stored energy to boost acceleration.

For example a 200 hp engine is no longer needed to accelerate a 3,000 lb car when a 100 hp engine, running at its “sweet spot” is used in conjunction with electric or hydraulic assist. Yet the 100 hp engine will consume less fuel. Toss in more efficient engine designs, control strategies such as start-stop, and weight reduction and you could have a far superior end result.

In fact, the only power needed is whatever it takes to power the car to the desired top speed up a certain grade, and powering accessories such as air conditioning. It takes a lot less than 200 hp for your average car.

Of course efficiency gains will depend on a number of factors. If a drive cycle is mostly cruising such as highway driving, the savings may not be there to justify the added vehicle cost. For commercial vehicles it’s a business decision. For consumers it’s less so but it’s still a factor.

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