The launch of the Mazda Xedos 9 in 1993 gave James May the chance to joke about Windy Miller when he reviewed the car for Car.
The cue for the gag came from the unusual use of the Miller cycle engine in the form of the 2.3 litre KJ-ZEM V6. All those years later the only thing I recall is the weak joke and nothing about the car or the technicalities of the Miller cycle engine. The reason I am bringing all this up is that Audi have been reported here to be reconsidering the use of the Miller cycle in the chase for better fuel stats.
You will find that C&D’s article follows their usual form in providing a clear layman’s interpretation of the Miller cycle engine’s principles, pros and cons. However, I have gone in search of enlightenment so I can present a stupid person’s guide to the Miller cycle. This does without the long lead-in required to understand where the Miller cycle differs from the bog standard Otto cycle which is the combustion process used in the majority of petrol engines.
What is the Miller cycle? First used in ships and stationary engines, it is really a variant or twist on the Otto cycle and not as different in principle from that as the Diesel cycle. The Otto cycle consists of the intake of petrol and air, compression, combustion and exhaust. Those are the same four steps used in both the Miller and Otto engine cycle. The Miller cycle saves work during the compression phase, needing a fifth less squashing effort to reduce the volume of the petrol-gas mix.
That might not sound like much but it adds up to a several-percent saving of energy. The second bonus is that you get more torque per litre of displacement compared to an Otto engine. In the case of the 1993 Mazda Xedos 9, the same power could be obtained from a 2.3 litre V6 as from a standard 3.2 litre V6. The saving there comes in the lower weight of the unit and the reduced friction losses inside the unit: narrower pistons and valves create less internal drag.
What put Mazda off pursuing this line of development – apart from the failure of the Xedos 9 to attract very many customers – was the cost of Miller-cycle engines which needed superchargers and intercoolers as part of the set-up.
Audi’s take on this is to use the Miller cycle on a four cylinder engine and to do it in the 2.0 litre range. They aim to get 188 horsepower, 236 lb-ft of torque and 49 mpg from the engine. Mazda needed 0.3 more litres and another two cylinders to get 213 hp (or 25 hp more). According to the official figures the Mazda only got 25 mpg on the combined US cycle (I converted to Imperial) Even taking procedural differences into account, that’s quite a saving which puts Audi’s engine up into the same efficiency level as clattery old diesel.
Here’s C&D’s attempt at the science part: “Typically, a Miller-cycle engine leaves the intake valve open on the compression phase longer than a normal engine. It operates in a similar fashion to the Atkinson-cycle engine, but uses forced induction (usually, but not always, a supercharger) to make up for the fuel-air mix forced out of the cylinder and through the open valve by the rising piston. The turbocharger helps the Miller-cycle engine to avoid power losses, even as it slashes fuel consumption by increasing the fuel-air mix in the cylinder after the intake valve closes.”
We ought to have noted this engine during our monthly theme on engines. The Mazda Miller was a 2.3 V6 which made it one of the smaller V6s, corresponding in displacement to the Ford Cologne 2.3 V6 and also overlapping in displacement with the five cylinder units of 2.0 and 2.4 litres capacity. Maybe we need to see how the Audi’s Miller 2.0 compares with Volvo’s quint 2.0 litre. I will come back to that.
The test of the Audi Miller engines will be in how they perform in real cars: they weight reduction is very helpful for FWD cars. But will the extra complexity come back to haunt Audi? All together: 1.4 litre TSI engine problems. And: for how long will Audi have exclusive use of the technology?
[The image above can be seen here at this rather informative blog.]