Miller Cycle

Today is the first of our seven fuel economy technologies that might have been deployed by Detroit on a mass scale but were not. The standard 4 stroke engines we have in our cars today use the Otto cycle. The Miller cycle was developed by Ralph Miller in the '40s and is also a 4 stroke cycle. It has been used commercially: the Mazda Millenia S had an engine using it. This Mazda engine was a 2.3 liter V6 that generated 210 horsepower and got 3.57 GPHM (Gallons Per Hundred Miles) highway driving.

The difference between a Miller cycle and an Otto cycle is in the compression stroke. The other 3 strokes are the same. In the Miller cycle, the intake valve is left open during the first 20% or so of the piston's rise up the cylinder. That means during the first part of the compression stroke, there is actually no compression. The fuel-air charge is forced back out of the intake valve instead of compressing. Then the intake valve closes and the remainder of the compression stroke does compress the charge. So that is the difference in the Miller cycle. The Miller cycle has unequal expansion and compression factors. The expansion phase uses the whole cylinder, while the compression phase uses only 80% or so of it.

If we build a Miller cycle engine so that it has the same size compression stroke as an Otto engine, it will be bigger. This is because the 80% of the cylinder that is used for compression in the Miller engine will have the same size as the whole 100% of the Otto cylinder. Looking at the diagram, the shaded area on the right shows the extra work that can be extracted from the Miller engine. The basic idea is that by lengthening the expansion stroke we give the engine extra time to extract useful work from the explosion that drove the piston down. By maintaining the same compression ratio, we do not have to worry about higher temperatures or pressures. But there is the problem of increased cylinder length. Miller cycle engines have the intrinsic disadvantage of lower power to mass ratios.

In practice, what is done is to build a Miller engine that is the same overall size as an Otto engine which means that for the same compression ratio the volume of cylinder that holds fuel air charge will be smaller. So for the same size, a Miller engine will be more efficient, but have less power, because there is less fuel-air mix to burn on each cycle. To compensate for this, it is common to add a supercharger to the Miller engine.

A supercharger precompresses the input fuel-air mixture, meaning it is denser. Although only 80% of the cylinder volume is useful for holding the fuel-air charge (because 20% got blown out) the denser charge means that the total amount of fuel is the same. Superchargers use some of the engine output power to run themselves, but even so a supercharged Miller engine can provide the same power output as a equal sized Otto engine while remaining about 15% more efficient.

So the question now is why has Detroit not invested in the Miller cycle? I think it is because of the costs and technical complications involved with the supercharging system. If Detroit wants to build Miller engines of the same size as their Otto engines, they need superchargers to get the same power output. The supercharger is the solution to the power to weight penalty Miller engines have. Until recently, the 15% gain on fuel economy was not worth the trouble and expense of including the superchargers. We will see if that begins to change in the future.