When Mazda introduced the 1995 Millenia, the big news surrounding the car wasn't about its luxury or price or performance, but that it was equipped with a new "Miller cycle" engine. Designed in 1947 by American inventor Ralph Miller, the Miller cycle had been a mainstay of the diesel engines in ships and pumping stations for decades, but the Millenia marked the first — and last — time it had ever been adapted for passenger-car use.
The Miller cycle is an offshoot of the mainstay Otto cycle and its four strokes. But in a Miller-cycle engine, the intake valve doesn't close at the beginning of the compression stroke. It doesn't close until the piston is nearly a fifth of the way through its travel up the cylinder wall. This is the engine design's ace up its sleeve; the design trick that makes the Miller cycle more efficient than its peers. Just why that's the case is the result of several factors coming into play at once.
With the intake valve open during the first fifth of its compression stroke, a Miller engine's piston will push some of air/fuel mix it sucked in back out through the intake port. Once the intake valve closes, there's less of an air/fuel mixture to compress in the cylinder, so less energy is spent compressing it — what engineers refer to as the "pumping loss." But even with more efficient compression, the smaller charge of air and fuel in the cylinder means less energy density, which means that the engine makes less power.
To counteract this, Millers are equipped with a supercharger. Pressurized air from the supercharger is run through an intercooler, which lowers the air charge's temperature — and also makes it denser, so more fits in the same volume. During the intake stroke, that charge of cold, compressed air rushes into the cylinder when the intake valve opens, filling it with a greater volume than could be sucked in by the simple downward movement of the piston. As the compression stroke starts, the supercharger's output keeps the cylinder pressurized until the intake valve closes, preventing the air/fuel mixture from being pushed back into the intake manifold. The piston pushing back against the air charge from the supercharger requires less energy than the piston compressing air in a closed cylinder, so pumping losses still remain lower than a traditional engine.
There's another way that the late intake valve closing combats pumping losses: On an Otto-cycle engine, the compression stroke starts when the piston is at the bottom of its travel; when the piston's wrist pin, the big end of the connecting rod, and the main journals of the crankshaft are all in a straight line. On a Miller-cycle engine, the delayed intake-valve closing means that the crankshaft travels through nearly 70 degrees of rotation before the intake valve closes, which moves the throw of the crankshaft's rod journal out of alignment with the other two. This creates a lever arm, that in turn means the crankshaft has a greater mechanical advantage when compressing the intake charge.
The net result of the Miller cycle's shorter compression stroke is an increase in the engine's expansion ratio. After combustion takes place at the top of the compression stroke, the expanding gases push the piston all the way down to the bottom of its stroke, allowing the air/fuel charge to burn thoroughly with little wasted energy. That the Miller-cycle engine creates power for the duration of its full stroke, but only has to spend energy squeezing the intake charge for four-fifths of its compression stroke, results in enormous torque for a given displacement.
But that's not where all of a Miller-cycle engine's advantages lie. Since they produce prodigious torque, a smaller-displacement Miller can be used in the same application as a larger traditional engine. During development in 1993, Mazda found that the displacement of its Miller-cycle V-6 could be reduced to 2.3 liters, while producing the same power as a theoretical 3.3-liter V-6 using the Otto cycle. The smaller displacement also resulted in lower drag — Mazda figured their Miller V-6 had 25% less internal friction than the larger engine because of its smaller pistons and valves. The combination of all those factors meant that the Millenia used 15% less fuel than it would have if Mazda had simply dropped in a bigger engine.
So why aren't more cars equipped with Miller-cycle engines? They're smaller and more powerful than Otto-cycle engines, but they are not — most critically — cheaper. By the end of its life cycle, the Miller-cycle V-6 in the Mazda Millenia produced only 10 more horsepower than the most-powerful version of the Mazda 2.5-liter V-6 on which it was based. Throw in the added expense of the supercharger, intercoolers, and their related hardware, and the fifteen-percent bump in mileage was overshadowed by the Miller's staggering cost of entry.
Yet all's not lost for the Miller cycle. Similar engines without the supercharger or intercoolers — known as Atkinson-cycle engines — have started sprouting up in hybrids, where the efficiency benefits of the shorter compression stroke and greater expansion ratio help those models achieve stellar mileage.