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  Hybrid with Atkinson-cycle

Not only at Toyota, but also Mercedes probably used the old Atkinson-patent to solve new problems, whereby, the Prius has been using it for some time already. It is almost the alternative concept to the currently, very often used downsizing with less cubic capacity and turbo-charging.

Although the Atkinson-cycle was occasionally used by Ralph Miller and the Mazda company with a compressor, nowadays, it can only be had without a compressor. Even the dynamic charging through the layout of the intake system is not used like it is in 'normal' petrol engines. The performance/capacity is also somewhat lower.

Have a close look at the small animation shown above, you'll notice that the opening time of the intake valve is distinctly longer. It closes very late in the compression stroke. Thereby, a portion of the mixture already present in the cylinder, returns back into the inlet manifold. The outcome of this is, less fuel-air mixture for the combustion process (and possibly, also less throttle loss).

Valve-lifting curves: left exhaust curve, right intake curve (normal/Atkinson)

Although in contrast to the layer charging, savings have also been made here, nonetheless, it is still a Lambda-1-mixture, which means that the exhaust gas decontamination process is as simple as that in the 'normal' petrol engine. Indeed, the possible performance/capacity of the engine is not exploited, which actually, is why this engine makes sense in combination with an electric-motor.

We don't know, whether Toyota actually gives the geometric compression ratio at 13 : 1. It may be even higher, which would be typical for an engine using the Atkinson principle. Thus, despite the late closing of the of the inlet valve, the engine can be operated close to the knocking limit. It would even be possible, to completely exploit the very high compression ratio, e.g., during the cold-running phase.

The variable valve timing makes this possible. Consequently, the cylinder head of the Prius III looks quite similar to other DOHC-petrol engines as well. One can indeed be curious, whether the process can be further optimized, particularly through variable compression, fully variable valve-lift and petrol-direct-injection. 12/13