Diesel: American dream
Volkswagen’s latest four-cylinder uses selective catalytic reduction and dual-circuit exhaust gas recirculation to get the Passat through Tier II BIN 5 emissions regs
- Published in Focus.
The Passat is VW’s latest diesel for the US. Its 2-litre engine offers 320Nm and 43mpg
Clean diesels for the US don’t get the headlines they did a few years ago – hybrids and electric vehicles get the attention now. Even so, VW has still been doing pretty well with compression ignition engines in the States.
In 2008 the company launched the Golf and Jetta compact cars with four-cylinder 2.0 engines developed for stringent Tier II BIN 5 emissions legislation. This unit has become the best-selling diesel engine in the US – more than a fifth of all VWs sold have one under the hood.
The firm wants to do better. Its plant in Chattanooga, Tennessee makes the midsize Passat sedan, which will help VW to increase market share, and sell many more diesels in the process. VW has developed a second-generation clean diesel for this application.
It generates the same 103kW/320Nm as the original but maintaining efficiency, driveability and BIN 5 compliance meant improving the induction and injection systems to reduce raw emissions, and the exhaust gas aftertreatment to take care of the rest.
“Our goal for this new Passat was to have the driving character that suits the midsize class and the fuel consumption of its compact class predecessor even though the vehicle is much heavier,” said VW’s head of diesel engine development, Jörn Kahrstedt. “This was a very ambitious goal.
“The three development priorities for optimum efficiency were the high- and low-pressure exhaust gas recirculation (EGR) systems and the charge air cooling. They all interact, and together we have an optimum solution regarding pressure losses. And the lower intake air temperatures help us to reduce raw emissions significantly.”
Better integration is one of the enablers for improved efficiency and performance. The high-pressure exhaust gas stream now passes through a cast channel in the cylinder head instead of an external pipe, with the feed coming from a port in the exhaust manifold. This improves response and reduces weight, packaging and cost.
The low-pressure EGR circuit, supplied by BorgWarner, is more efficient and compact than before. The filter located in the feed pipe, which prevents particulates from hitting the turbocharger compressor wheel, is now a woven design: backpressure has been cut by as much as half.
Because the charge air cooler is now watercooled, the EGR cooler’s capacity could be downrated; reducing the number of passageways and increasing their diameter reduced pressure losses too.
“The EGR cooler’s performance has been reduced so much that it still delivers optimum temperatures without a bypass valve,” said Kahrstedt. “Now low-pressure EGR can be used across a broad operating map. The advantages are greatest in areas of very high EGR rates.”
The watercooled charge air cooler is VW’s first diesel application of the technology, although some turbocharged gasoline engines have them.
This one is aluminium instead of plastic though, because of the greater loads, and on a separate cooling circuit, including heat exchanger and electric coolant pump. The pump only runs once coolant reaches optimum temperature, improving thermal efficiency.
The turbocharger is now much more efficient at part-load, improving fuel consumption and driveability. The turbine housing is integrated into the exhaust manifold casting, reducing weight, cost and thermal inertia. To improve flow rates, the turbine inlet diameter was increased, and the variable geometry turbine blades are now straight rather than curved.
