AUTO TECH: Exhaust-heat recirculation a simple concept

Hey there, time traveller!
This article was published 15/01/2010 (5833 days ago), so information in it may no longer be current.

LOOKING back at all the technologies introduced on vehicles in the past year, many stand out.

Ford’s EcoBoost system with their twin turbocharged engine produces amazing power with good fuel economy from a small engine. Porsche’s PDK computer-controlled manual/automatic gearbox offers exciting performance with a simplicity anyone can operate. Volvo’s City Safety is only a small glimpse of the safety systems they have introduced, such as the Blind Spot Information System and of course their concept safety cars that lead the industry in looking forward.

Even the aftermarket has had exciting offerings, such as enhanced fuels and low-rolling-resistance tires.

With all this in mind, there is one technology that stands out for me for several reasons and it is currently available on the Toyota Prius and Lexus HS 250h. It’s called exhaust-heat recirculation.

The concept of this technology is simple. Take the heat from the exhaust system that is normally wasted and convert it into heat we can use during cold starts and engine warm-up periods. Currently it is only used on these two hybrid models, but there is no reason it couldn’t be used on any production vehicle with very little modification.

Cold-engine start-up performance is crucial to long engine life and good fuel economy. It takes a very rich air/fuel mixture to ignite in cold engine cylinders. Much of this fuel is wasted and dilutes the oil film on the cylinder walls, causing more engine wear. As the engine warms up, additional fuel is needed to keep the engine idling smoothly. Only when the engine is fully warmed up will optimum fuel economy be achieved. This is why good thermostat operation is important for fuel economy. If the thermostat opens too soon, it takes longer for the engine to warm up and more fuel is wasted. Exhaust heat recirculation reduces heat waste by warming engine coolant during cold start-up. It also heats up the passenger cabin quickly.

When we look at the internal combustion engine, it is notoriously inefficient. Even with all the performance improvements in the past couple decades, engines are only about 35 per cent efficient in turning the heat energy of the fuel into mechanical energy. Some is lost to friction, but the biggest loss is heat out the exhaust pipe. Turbocharging increases engine efficiency because it uses exhaust heat to compress the intake air. Combine this with high-pressure gasoline direct injection and the efficiency peak could go up to about 45 per cent, but both add complexity and cost to the engine design. Most engines seldom operate at their peak efficiency, so average efficiency could be as low as 20 per cent.

The diesel Otto cycle is more efficient than current gasoline engines because the diesel engine operates with a much higher compression ratio and the injection cycle of fuel creates more constant cylinder pressures rather than the peaks of a gasoline engine. This can increase efficiency above 40 per cent and the addition of turbocharging and high-pressure fuel systems takes it even higher. The disadvantages of diesel engines continues to be higher cost for parts to withstand the high-compression pressures and the higher cost of precision fuel injection parts. There is an additional penalty in the added weight of the engine, but a diesel engine does convert more of its fuel into usable power.

As you can see, even with the most efficient engines there is still a large percentage of waste. Any way we can recapture some of that waste will pay big dividends in average engine efficiency, and exhaust heat recirculation does just that.

Volkswagen used to use the engine exhaust to provide interior heat for their vehicles back in the ’60s and ’70s. Unfortunately, the systems were air-to-air heat exchangers and didn’t work well most of the time.

The current systems are air-to-liquid heat exchangers, where the exhaust gases heat the engine coolant. There is no danger of exhaust gases leaking to the interior, and existing coolants can be used in the system. It doesn’t need an additional waterpump to circulate the coolant, although an electric pump could be added to better control the temperature of the coolant coming from the system.

No documentation on how much the system improves engine efficiency on the Prius, but I can tell you that the engine and passenger compartment do warm up much faster. I look forward to seeing it on more vehicles.

Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net