Rudolf Diesel (1858- 1913), the inventor of the compression engine we now know as the diesel engine, said: “The automobile engine will come, and then I will consider my life‘s work complete.” Rudolf‘s early engines only used about 26% of the energy content of the fuel they burnt. Research suggests that less than 50% of the potential energy in a diesel engine‘s fuel is used to power the vehicle.

Energy losses

In 2010, Oil & Gas Science and Technology – Revue d‘IFP Energies Nouvelles published the article, Energy and Exergy Balances for Modern Diesel and Gasoline Engines. This suggested only approximately 30% of the energy content of the fuel burnt in a diesel engine was used for work, i.e. to move a vehicle. In 2013, the International Council on Clean Transportation (ICCT) suggested that 43 to 44% of the energy in diesel fuel was used for useful work.

For many years, engineers have been striving to use this wasted energy to improve the overall efficiency of the engine and the vehicles they are fitted to. Turbochargers, using the energy in the exhaust gases, is one way this otherwise-wasted energy is used. Another is using it to drive a turbine that can increase the engine‘s power output, a turbocompound engine.

History

The need to increase the power output of internal combustion engines, particularly in aircraft, was highlighted during World War 2. Several aircraft engine manufacturers developed prototypes, including RollsRoyce in 1941, but none went into serious production. Post-war, some development continued with Napier producing the Napier Nomad in 1949 and a variation, the Wright DuplexCyclone, used in many commercial passenger and military aircraft, such as the Lockheed Super Constellation and Boeing B29. However, the rapid development of pure jet and jet turbine engines resulted in the development of TCEs for aircraft all but ceasing. In 1987, Scania announced it was producing a TCE variant of its popular 11-litre engine. Scania claimed that this engine would have a power output of 298kW (400hp) compared with 265kW (356hp) from a similar engine without turbo-compounding. Scania also claimed that efficiency of the TCE was 2% greater than the non-turbo-compound engine, with a resulting reduction in fuel consumption of about 5%.
In 2007, Detroit Diesel announced a TCE engine, the DD15, also claiming approximately 5% improvement in fuel consumption.
In 2017, Volvo launched its TCE, the D13TC. The claim was that this engine could deliver 6.5% fuel savings over a similar engine without turbo-compounding. In 2019, a second generation of this engine was launched, delivering a potential for a further 3% in fuel savings.

How does a turbocompound engine work?

A typical TCE uses some of the energy content present in the exhaust gases after combustion is completed to drive a turbine. This turbine is connected by a gear train to the engine‘s crankshaft, supplementing the energy produced by the combustion process at the crankshaft to power the vehicle.

Instead of connecting directly to the engine‘s crankshaft, some New Zealand Trucking July 2020 35 turbo-compound engines use the energy to power an alternator, producing electrical energy that can charge batteries and/or power on-board auxiliary equipment.
Although positioned in-line of the exhaust system, TCEs have shown not to increase exhaust-system back pressure. There is a marginal increase in engine weight, however.

Why turbocompound engines?

During development, and in-field testing, TCEs have shown to increase the energy produced by a diesel engine that is used to drive a vehicle and its accessories. These same tests have also shown that a TCE truck can use approximately 5% less fuel for the same task, compared with a similar engine truck that does not have a TCE.