 Miller-Timing
| Miller-Timing: lower filling - higher compression |
We've already written a great deal about the two inventors, Atkinson and Miller. Perhaps not so much about the former, but this is actually the most appropriate place for discussion about the latter. By redesigning the
crankshaft drive, Atkinson changed the working and exhaust strokes compared to the intake and compression strokes. Read more about this in our book Combustion Engines.
In stark contrast to previous explanations of power enhancement, Miller timing refers to the very early closing of the intake valves. We've always discussed this in conjunction with variable valve timing. However, the basic
principle also works with fixed valve timing, as the patent dates back to 1957.
| By the way, closing the intake valves very late can have the same effect (picture above).
|
You're probably uneasy at the thought of closing the intake valve so early. Where would the power come from then? In fact, Miller timing engines tend to operate with larger displacement and yet seem like slackers. An
example would be the 1.8 liter four-cylinder in the Toyota Prius. What's notable about this engine, however, is its high geometric compression ratio.
It has to be that way, because without it, there would be no visible benefit. But with its low filling, it at least achieves the same performance as a regular gasoline engine. What's great, of course, is that the variable valve
timing also allows control along the knock limit in the Prius. When the engine is cold, the intake valves could even be allowed to close significantly later, despite increased compression.
But, as I said, the image of a poorly accelerating engine remains. The Prius, of course, has its electric drive, which can compensate for this relatively quickly. This also saves on the enrichment, which can sometimes be
quite expensive in terms of consumption. Of course, with a gasoline engine, you can't simply open the throttle valve to accelerate.
That was actually a lesson in upsizing rather than downsizing, wasn't it? You may have already noticed that many things are possible, but one thing isn't: lowering the compression ratio. Please note, because compression
can not only be responsible for high performance, but also for low fuel consumption and thus CO2 emissions.
You can therefore reduce the fresh gas with increased compression, whereby the geometric has lost some of its importance compared to the past due to the many control options. Variable valve timing allows you to adjust
it downwards during operation, and it can be adjusted upwards with increased boost pressure.
The worst way to reduce the fresh gas supply is with the traditional throttle valve, which is directly actuated by the accelerator pedal. A particular disadvantage is that their most unfavorable position occurs most frequently in
real driving operation, namely the reduced fresh gas supply at reduced torque. At full throttle or idling, their effect is not so detrimental.
This is much better with e-gas, because one can choose the design whereby one wants to slow down the development of power and torque at that particular moment. Just one example: If you were to achieve less boost by
reducing the ignition, the restart would be more spontaneous because the advanced ignition acts significantly faster than a reopened throttle valve.
However, the potential elimination of the throttle valve is being used so heavily as an argument for fully variable valve timing that one wonders whether it doesn't actually perform the same function, meaning that the
throttling, which is undoubtedly detrimental to efficiency, is simply relocated. But, the additional constriction at the intake valves that a throttle valve creates is then eliminated.
| 
