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Why does Volkswagen have a water-cooled exhaust?

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Volkswagen's 1.8L Turbocharged Engine Has A Water Cooled Exhaust.

From VW's Press Kit On The EA888 Engine:
"Volkswagen has a long track record of producing small-displacement engines that achieve high levels of performance and efficiency, and the new EA888 engine is evidence of that deep experience.

Like its predecessor, the engine has a cast-iron cylinder block and an aluminum-alloy cylinder head which now has the exhaust headers cast integrally. The cast-iron crankshaft runs in five main bearings. The twin overhead camshafts are chain driven, and there is variable cam phasing on the intake side.

With a bore and stroke of 82.5 by 84.1 mm and a displacement of 1,798 cc, the 1.8T engine packs quite a punch, with peak output of 170 horsepower at 4800 to 6200 rpm, matching the horsepower rating of the larger 2.5-liter five cylinder, but delivered at 700 fewer revs. The new engine offers seven pound-feet more torque, produced at just 1500 rpm—a whopping 2750 revs less than the outgoing 2.5’s torque figure. The torque curve is strong and flat, with the peak figure being delivered from 1500 to 4750 rpm.

The new engine also returns much better EPA estimated fuel economy figures than the 2.5-liter engine. While the actual fuel economy varies by model, the Jetta equipped with the 1.8T and a six-speed automatic transmission now gets 36 mpg on the highway, up from 31 mpg for the 2.5-liter engine. At the same time, the city mileage improves from 24 to 25 mpg, and the overall EPA estimated combined fuel economy is now 29 mpg compared with 26 on the outgoing five cylinder. As well as offering better fuel economy, the new engine’s additional low-speed torque has enhanced the performance of the cars fitted with the engine: the manual transmission Jetta 1.8T now goes from 0 to 60 mph in a manufacturer estimated 7.3 seconds, an improvement of 0.7 seconds over the 2.5-liter model.

One of the keys to this improved performance and fuel economy is the reduction of internal friction. Several innovations contributed to this reduction including; the use of a new piston coating; the two balance shafts that counteract the second-order inertial forces run in roller bearings; utilizing a low-energy oil pump; and using a highly precise electric system to control the oil-jet cooling for the piston crowns.

Turbocharger and Direct Injection Improvements
The turbocharger in the EA888 is an all-new design that develops a relatively high boost pressure of up to 19 psi (1.3 bar). Key features of the turbocharger include; a turbine wheel made from a new alloy that can withstand exhaust temperatures of up to 1796 degrees Fahrenheit; a pulsation damper; the addition of an oxygen sensor mounted directly upstream of the turbine wheel; use of a compressor wheel machined from solid; and a new electric wastegate actuator that adjusts the boost pressure, when power is not needed, to help reduce fuel consumption.

Thermal Management
Thermal management is key to helping ensure maximum efficiency. The thermostat keeps coolant temperature between 185 and 225 degrees Fahrenheit, influenced by load and engine speed, to achieve the ideal balance between minimal internal friction and temperature management. By directing exhaust gases through the water-cooled exhaust manifold, they are approximately 160 degrees cooler by the time they reach the turbocharger.

Under full load, heat is significantly reduced thanks to the integrated cylinder head and water-cooled exhaust manifold. This new design eliminates the need to enrich the air/fuel mixture under full load, resulting in a reduction in fuel consumption of approximately 20 percent when driving at highway speeds."

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Duration 06:29
Posted Jun 22, 2017
Series Automotive
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