“EGR (exhaust gas recirculation) systems now in use on diesel engines were used to meet emissions regulations. In gasoline engines, we think they are an ideal way to meet stricter fuel economy standards,” explained Martin Bauer, Development Director for engine-mounted components at Mahle Behr USA, in an interview with Automotive Engineering.
EGR systems divert some of the engine-out exhaust gas and mixes it back into the fresh intake air stream. Mixing exhaust with the intake air lowers combustion temperatures and rates. This improves emissions by reducing the formation of NOx. It also reduces the knock limit, providing better fuel economy through higher compression ratios and/or spark advance.
Cooling the exhaust before mixing it into the intake stream in a special heat exchanger further improves emissions and the knock limit. Bauer estimates that cooled EGR could improve the average fuel-economy from 2% to 5%, as measured on current drive cycles used by regulatory agencies.
“We especially see improvements from cooled EGR for downsized gasoline engines that are turbocharged and use direct-injection,” he said. That is because such engines are designed to run at higher loads and speeds. That is the operating region knock limit and mixture enrichment seriously affects fuel economy.
Another potential application is engines used in hybrid electric vehicles. Typically, these Atkinson-cycle engines also run at high loads and speeds, though they are not typically turbocharged or use direct-injection.
To understand better the benefits from cooled EGR, Mahle Behr ran a series of tests on a three-cylinder 1.2-L turbocharged direct-injection gasoline engine with a 9.3:1 compression ratio. The test engine produces 120 kW (161 hp) peak power and 286 N·m (211 lb·ft). The engineers compared two types of cooled EGR: a high pressure system that diverts exhaust before it powered the turbocharger turbine, and a low pressure system that diverts exhaust from downstream of the turbocharger turbine.
The cooled EGR device is a laser-welded fabrication similar to those the company supplies to diesel applications. It measures 2 x 1.5 x 8-in (50.8 x 38 x 203-mm) and typically weighs 1.4 lb (.63 kg). The unit is constructed of corrosion-resistant 300- or 400-grade stainless steel.
“We found that the low pressure cooled EGR showed improvements over the entire speed/load operating map,” said Bauer, with a maximum improvement at the highest speed and load of 9% better fuel economy. The high pressure cooled EGR showed even better maximum reduction, with 18% better fuel economy at the highest speed and load.
However, he tempered this finding by saying that the average fuel economy as measured on four different drive cycles (NEDC, WLTP, Artemis, and MPT-HLC) was actually better for the low-pressure-cooled EGR.
“We predict that the newer driving cycles, such as WLTP or Artemis, will have higher shares of high load and high engine speed, meaning cooled EGR will provide benefits in better fuel economy as a result,” he said.