Turbo-compounding is quite an old technology to improve power and efficiency in reciprocating engines that has received renewed attention in the last few years. It uses a turbine to recover energy from exhaust gases, like a turbocharger. Rather than using that energy to drive a compressor, it is used to directly add power to the engine’s output shaft. Thermodynamically, it is a combined cycle engine. The piston engine is the first cycle, and the second cycle is a gas turbine. A combined cycle is able to achieve a higher thermodynamic efficiency.
Older turbo-compounding engines mechanically coupled the turbine to the engine’s output shaft. The new generation of turbo-compounding engines use the turbine to generate electricity, which can then be used in a number of ways.
The History of Turbo-Compounding
In the 1950s, turbo-compounding was used to increase the efficiency of reciprocating aero engines for commercial aircraft. Efficiencies of up to 40 percent were achieved. The power-recovery turbine was mechanically coupled to the propeller using a continuously variable transmission, such as a Beier variator. As these engines developed, it became apparent that the turbine section was far simpler, more reliable and more efficient than the piston engine. It made sense to get rid of the reciprocating part of the engine entirely, leading to the development of turboprop and turbojet engines.
Within automotive engines, it has been much more common for any mechanical energy recovered from exhaust gases to be used to drive a compressor on the air intake—a turbocharger. However, some large engines for heavy vehicles have used turbo-compounding over the years. Mitsubishi was one of the first, using turbo-compounding on the 10ZF tank engine in the 1960s. In a similar application during the 1980s, Cummins used it for the V903 engine in the Bradley Fighting Vehicle. There have been more recent commercial uses in heavy truck engines such as the Volvo D12 and Scania DT12, which have both avoided complexity by using fluid couplings instead of mechanical continuously variable transmissions.
Electric Turbo-Compounding (ETC)
A limitation of mechanical turbo-compounding is that at low engine power, the backpressure caused by the turbine can actually reduce engine efficiency and the completeness of combustion, leading to increased emissions. Turbo-compounding always reduces the efficiency of the engine itself. At high powers, the turbine recovers more energy than is lost, leading to a net efficiency gain. At low power, this reverses and reduces efficiency. Overcoming this may require diversion values, adding yet more complexity.
Electric turbo-compounding (ETC) can avoid the complexities of continuously variable transmissions and issues with back pressure by using the exhaust powered turbine to generate electricity. Since 2014, Formula 1 cars have used ETC combined with an electrically powered compressor on the intake compressor. When the engine is running at high power, excess energy in the exhaust gases is used to generate electricity. This can be used to power the turbocharger, add power to the wheels or charge a battery, as required. When the engine is not generating enough power or exhaust gas pressure to power the turbo, battery power is used to keep the turbo spinning, preventing any turbo lag when the power demand increases.
ETC is also starting to be used for large commercial engines, for heavy vehicles, and for both gas and diesel-powered gensets. The increased simplicity of ETC when compared to mechanical turbo-compounding means it becomes much more attractive for those applications. While in the past it was difficult for vehicles to use all the electrical power produced, hybridization means that it is now readily used. One of the leaders of this technology, Bowman Power, has applied ETC to engines including CAT, Cummins, INNIO and Wärtsilä.
ETC for Power Generation
The most efficient heat engines are combined cycle power plants with a gas turbine delivering waste head to a steam turbine second cycle. They achieve efficiencies of over 60 percent. These work well at power outputs of several MW, but the high capital costs make them uneconomical at smaller sizes.
ETC can’t match these efficiencies, but it’s suitable for much smaller gensets, typically in the 150kW-2.5MW range where it can increase the efficiency of diesel- and gas-powered piston engines by 4 to 7 percent. The upper end would be for older or lower powered engines while 4 percent is still achievable for modern turbocharged engines.
For most genset installations, ETC doesn’t actually add to the per kW cost of the system since the increase in power offsets the additional cost. Combined with the improved energy efficiency, that means a much more economical system.
“ETC is very much a proven technology, with 22 million operating hours across 800 systems,” said Mike Essex, Bowman Power head of marketing.“Our third generation of ETC, released in 2017, reduced costs by 50 percent and is now being used for landfill gas, waste water treatment, the rental market and other sectors. We are working on a development program that will bring a further reduction in cost. ”
The increased efficiency of an engine fitted with ETC will obviously mean less CO2 emissions. The impact on other emissions can be even more dramatic. Increasing the back pressure on cylinders reduces fuel short circuiting (methane slip). Bowman has demonstrated a 32 percent reduction in unburned hydrocarbon emissions for a gas-powered generator. This can have dramatic implications for greenhouse gas (GHG) emissions since methane is 30 times more powerful as a GHG than CO2. Other pollutants that affect air quality can also be reduced, such as particulate matter (soot, ash and aerosols), oxides of nitrogen (NOx), sulphur oxides and carbon monoxide. Although these are emitted at approximately the same rate per unit of fuel burnt, they are lower relative to the energy produced.
ETC is an appealing technology for large reciprocating engines because it can significantly increase efficiency and power density with only a small increase in cost and complexity. It may be set to become as common as turbocharging on modern engines.