Volkswagen’s 1.5L Miller Cycle Engine: How Does it Work?

A few years back, Volkswagen revealed a new 1.5L turbo engine that used the Miller Cycle to improve fuel consumption and low-end torque. Just a couple of weeks ago, we told you that the EA211 1.5L TSI Evo engine might be coming here. But what’s the Miller Cycle? Isn’t it that Ducati dealer down the road? And how is this different from the Budak engine that VW just put in the Tiguan? Why is VW trying so many different things lately?

Well, the answer to the last one is that as automakers get really good at making gas engines, they need to find different ways to operate, or else just keep making tiny improvements on what they’re already doing. In this case, VW is trying different ways to work to make their engines more efficient without sacrificing power. The Miller Cycle is one of those.

The Miller Cycle was patented by Ralph Miller way back in 1957, though he started working on it years before. If you’ve heard about it before, it was probably because of that other bastion of strange engines Mazda. They used the Miller Cycle on the 2.3L V6 of the Millenia during the late ’90s.

Now bear with me for a bit, cause this part isn’t the exciting part, but it’s stuff you need to know. The Miller Cycle is a modification of the Otto Cycle. The Otto cycle is what we call the thermodynamic cycle that makes a modern car engine work. It describes the cycle of intake, compression, expansion, and exhaust that make up the four-stroke engine.

Intake valves open, piston moves down and pulls air in. Intake valves close as the piston hits bottom and the piston comes back up. Compressing the charge. Then spark, which forces the piston back down. That’s the power stroke. Then the exhaust valve opens and the piston pushes the combusted air out. It’s pretty simple, and it makes cars go. The old suck, squish, bang, blow of the ICE.

Ralph Miller changed that. Just a tiny bit. But enough to make a big difference.

When the piston hits the bottom of the intake stroke, the intake valves are supposed to shut. Otherwise, they’re squeezing out that sweet, sweet gasoline/air mix. In the Miller version, the intake valve stays open for a little bit of the compression stroke. About the first 20-30 percent. So doesn’t that just make less power?

No. It makes things get efficient.

Because the Miller Cycle needs boost to work. Originally with a supercharger, but Volkswagen is getting things done with a turbo.

Miller Cycle – Mazda

Everyone’s heard of compression ratio. How much the air gets squished by the piston’s compression stroke. Higher values are generally associated with more power and more efficiency. But it’s the expansion stroke that’s the real hero. The more time the engine spends with the pistons being forced down, the better. Because the whole time the air and fuel are burning and expanding, they’re making power. But compression and expansion ratios are effectively the same. Because the piston has to go up and down the same amount.

Leaving the intake valves open means that for the initial 20-30 percent of the compression stroke, nothing is actually being compressed. There’s not much holding the piston back over that distance. It allows for the expansion ratio of the engine to be much higher than the compression ratio. Making sure more work is extracted from the air/fuel mix.

Without boost, it’s just about the same as the Atkinson cycle. No, not the no-carb diet, the engine cycle that hybrids use. Because they get the benefit of the bigger expansion ratio, then can make up for the power losses with their electric motors instead. Plus nobody expects their Prius to be quick.

In order to make sure the engine makes enough power, forced induction is necessary. So for VW, a turbo compresses the air. Then, and this is important, the air is sent through an intercooler. That lowers the charge temperature. Because the effective compression ratio inside the engine is lower, that charge stays cooler than it would in a normal Otto-cycle engine. That lets the engine computer advance ignition timing to much earlier in the cycle than it could with a normal engine. Advancing timing increases efficiency even further as it gives the charge even longer to expand on the power stroke.

In a conventional gas engine, much of the expansion of the fuel is wasted when the exhaust valves open. The Miller cycle, by making the expansion cycle longer than the compression cycle, reduces that waste. Making more power, and making better use of the fuel it burns. That’s how VW can get 150 hp and 184 lb-ft out of a 1.5L four, and is aiming at 180 hp. More impressively, that’s how it can get 48 mpg combined in a Golf with that kind of power.

Mazda Millenia by TTTNIS – Own work, Public Domain, Link