Hot Heads: How and Why VW Cools its Exhaust Manifolds

The new EA888 engines that come in the Passat, Beetle, and old-body Jetta, as well as the new Tiguan, have some pretty neat features. Some of them are to make more power, and some–like the 2.0-liter Tiguan’s modified Miller-cycle–are to boost fuel economy.

There are electric wastegates, a lighter cylinder block, plastic oil pans, thinner cylinder walls, aluminum fasteners, and more. But the coolest on the third-generation of the engine is the integrated exhaust manifold with liquid cooling.

Why on earth would an automaker want to cool the exhaust? Especially on a turbocharged engine? This is how (and why) it works.

It starts with an exhaust manifold that’s integrated into the cylinder head. Instead of the four separate exhaust ports you’d expect to find on a four-cylinder engine, there are just two holes in the center of the head. That’s because the exhaust passages meet up inside the head instead of outside, in a separate manifold.

Ditching the old-fashioned exhaust manifold is good for a few reasons. The turbo is bolted directly to the head, putting it almost on top of the exhaust valves, helping the turbo spool faster. That means better throttle response and the hotter exhaust gasses can lead to improved power output. On the other side of the equation, the catalytic converter can be mounted extra close to the engine, which reduces cold start emissions. That’s an alluring combination and it’s why VW has been using an integrated exhaust manifold since 2013.

But that extra hot exhaust can be bad for the cat once the engine’s up to temperature. The heat can cook the insides of catalytic converter and cause it to fail, which is less good for emissions.

So VW (and everybody else) needs a design that keeps the cat and the turbo close to the exhaust valve, but that doesn’t cook with high exhaust temps. Oh, and it needs to warm up even faster, because getting engine and cat up to temp are the most important parts of emissions testing.

So the engine needs to be hotter and cooler, depending on how long it has been running. Sure, no problem.

Enter the cooled integrated exhaust manifold.

Adding cooling to the exhaust manifold has some interesting effects. When you start the engine, the coolant starts sucking up heat directly from the exhaust. That warms the engine up faster. When the engine gets up to temperature more quickly, you get the engine to run in its most efficient modes more quickly. One of the happy side effects of the setup for people of the north is that the car starts pumping heat into the cabin more quickly.

Once the engine is warm, the coolant bath works to cool the exhaust. With a turbo and a close-coupled catalytic converter, too much exhaust heat is an issue. That’s why some ’80s supercars had that funny Slow Down warning light. It was telling you the cats were overheating.

Regular engines reduce exhaust temperature by adding more fuel. Rich fuel mixtures burn cooler but, as you might expect, that hurts your fuel economy. With the coolant bringing down exhaust gas temperature, the engine can run leaner. That saves fuel when you’re cruising, and the cool temperatures prolong the life of the cat, the turbo, and your oil.

Best of all, it doesn’t take much special plumbing. It’s just normal coolant, run through the radiator like normal coolant would be. But by circulating it around the exhaust manifold, Volkswagen has made a vehicle that is cleaner, more comfortable, and thriftier at the pump. And you don’t have to worry about rusted out exhaust manifolds 10 years down the road.

So that’s how VW’s coolest exhaust tech works, and why they use it. Just another way automakers are pushing internal combustion engine tech to levels that they only dreamed about a few decades ago.