Turbo-compounding has historically been applied to reciprocating aero engines, with the power-recovery turbine connected to the output shaft. It was quickly realised that the turbine section of the engine could completely replace the piston engine. It was more efficient at recovering energy from the hot gas, as well as being more reliable. Turbo-compounding piston engines quickly gave way to turboprop and turbojet engines.
Since the 1950’s, most piston engines that recover energy from exhaust gases have used that energy to drive a turbocharger. However, there is renewed interest in turbo-compounding using electrical generation rather than turbines mechanically coupled to the output shaft.
Since 2014, Formula 1 cars have used turbo-compounding engines with a turbocharger, connected to an electric motor/generator. Outside motorsport, electric turbo-compounding (ETC) has the potential to significantly improve the efficiency of gas and diesel-powered gensets, as well as large vehicle engines.
The original form of turbo-compounding transmitted power from the turbine mechanically. Some aero-engines used it to drive a separate propeller, while most transferred the energy into the main output shaft using a continuously variable transmission. For example, the Nomad engine (see diagrams) used a Beier variator.
High-efficiency aero-engines quickly evolved from turbo-compounding into turboprop engines. These proved to be mechanically simpler and more reliable, albeit more expensive to manufacture. There did, however, continue to be occasional use of turbo-compounding in heavy land vehicles. Later systems avoided the need for continuously variable transmissions. However, they still required two-stage-reduction gearboxes, combined with fluid couplings, adding considerable mechanical complexity to the engines.
A further issue with mechanical turbo-compounding is that at low engine loads, insufficient pressure is produced to drive the additional turbine without a negative impact on engine performance. The resulting backpressure can reduce fuel efficiency and increase emissions. Diversion valves may be needed to mitigate for these effects, adding more complexity.
HIGH-EFFICIENCY POWER GENERATION
Using turbo-compounding to generate electrical power removes the need for complex mechanical power transmission and allows the load to be reduced when required to maintain an acceptable backpressure. In the past, it was difficult for vehicles to use the significant amount of electrical power produced.
However, with widespread hybridisation of vehicles, finding a use for this power is not an issue. ETC is now ideally suited to large diesel engine vehicles, both land-based and marine, and stationary power generation.
The highest efficiencies for heat engines are typically obtained from combined cycle power plants, which use a gas turbine for the first cycle and a steam turbine for the second cycle. This type of power plant can achieve real-world efficiencies of over 60%. However, such plants are only economical for significant power outputs, typically several MW, with high capital costs, fixed installations and significant project duration.
Turbo-compounding is suitable for much smaller engines, as demonstrated by its use in Formula 1. Within power generation, the focus is on gensets in the 150kW-2.5MW range, where it is possible to improve the efficiency of diesel and gas-powered piston engines by 4-7%. “For modern turbocharged engines, we can achieve a 4% improvement in fuel efficiency. For older engines, or low-energy gas applications, we can achieve a 7% increase in efficiency. It primarily depends on how much energy there is in the exhaust, and the exhaust temperature,” says Keith Douglas, head of performance at Bowman Power.
Although the backpressure caused by turbo-compounding reduces engine efficiency, the turbine recovers approximately 2.5 times more energy than is lost, leading to a net efficiency gain. As an example, consider a 1MWe engine with a baseline of 38% electrical efficiency. Applying the ETC system while keeping the fuel flow constant would result in an approximate loss in engine load due to pumping losses of 40kWe, giving 36.5% electrical efficiency for the engine. But the ETC system would then generate approximately 100kWe, leading to a net gain of 60kWe and 40.3% electrical efficiency in total.
The main competing technology is Organic Rankine Cycle (ORC), which are also able to generate useful work from exhaust gas and may be fitted to gensets. However, ORC is significantly more complex and expensive, while the organic fluids used may also be highly damaging if they are released to the atmosphere, such as CFC’s and HCFC’s. ETC gains efficiency from a multiple-stage expansion, with a system that is said to be much simpler and easier to install.
Bowman Power’s ETC systems have been fitted to gensets produced by many major OEMs. “ETC is very much a proven technology, with 22 million operating hours across 800 systems,” explains Mike Essex, head of marketing at Bowman Power. “Our third generation of ETC (2017) reduced costs by 50% and is now being used for landfill gas, wastewater treatment, the rental market and others.”
For typical genset installations, the addition of an ETC system does not add any significant cost in terms of unit cost per kW. The increase in power offsets the additional cost, resulting in a more economical system with payback in as little as 12 months.
“We are always careful to ensure that by increasing power output, we don’t overload the genset,” adds Douglas. “Older engines can achieve efficiency and power gain, while newer engines are operated at the same power with only an efficiency gain. This is done to ensure firing pressures are kept within the engine’s limits. Our OEM engine data allows us to increase power where possible. The only significant maintenance is a bearing change every 30,000 hours, aligned with host engine service.”
Improved fuel efficiency also results in lower CO2 emissions – important to climate change.
When it comes to local air quality, other emissions are more significant, such as particulate matter, oxides of nitrogen, sulphur oxides, carbon monoxide and unburned hydrocarbon (UHC). These emissions can have serious public health implications and, in the case of UHC, may also act as greenhouse gases. Bowman recently demonstrated a 32% reduction in UHC emissions for a gas-powered generator – achieved due to the increased backpressure on cylinders, which reduces fuel short-circuiting (methane slip). Although other emissions are typically the same per unit of fuel as without ETC, they are lower relative to the energy produced.
Electric turbo-compounding enables an improvement in engine efficiency with only a modest cost/complexity increase. As with turbo-charging, it looks set to become a standard feature of modern engines.