Exhaust heat recovery system

In transportation, an exhaust heat recovery system turns thermal losses in the exhaust pipe into energy. This technology seems to be more and more of interest by car and heavy-duty vehicle manufacturers as an efficient way to save fuel and reduce vehicles’ CO₂ emissions. This technology can be used either on a hybrid vehicle or a conventional one: it produces either electric energy for batteries or mechanical energy reintroduced on the crankshaft.

Principle

Thermal losses of an internal combustion engine

Even if current engines consume less fuel than they used to, the thermal efficiency of an internal combustion engine has not really increased since its creation. The peak efficiency reached by a 4-cycle Otto cycle engine is around 35%, which means that 65% of the energy contained in the fuel is lost as pumping losses, friction losses, cooling losses, exhaust losses and accessories. High speed Diesel cycle engines fare a little better with around 45% peak efficiency, but are still far from their Carnot efficiency, and hence 55% of the fuel energy content is lost.

Thermal losses in the exhaust pipe

Inside the exhaust pipe of an internal combustion engine, energy losses are various: thermal, kinetic, chemical and latent heat. Most important energy parts are located in the thermal and kinetic losses, the two others are negligible. Kinetic losses can be recovered through a turbocharger or a turbo-compound.

Exhaust heat recovery technologies

Rankine

A typical Rankine Cycle is a thermodynamic cycle that uses a fluid and works thanks to four reversible processes. In transportation, Rankine cycle systems vaporize a pressurized fluid, thanks to a steam generator located in the exhaust pipe. As a result of the heating by exhaust gases, the fluid is turned into steam/vapor. The pressure will then drive the expander of the Rankine engine, which could be a turbine as well as a volumetric expander. This expander can be either directly tied to the crankshaft of the thermal engine or linked to an alternator to generate electricity.

The Fluid used in Rankine Cycle Engines can be a “humid” fluid (such as water) or a “dry” organic fluids. The choice of the fluid depends in particular on the running temperature of the system. Researchers at Loughborough University and the University of Sussex, both in the UK, also have concluded that using waste heat from light-duty vehicle engines in a steam power cycle could deliver fuel economy advantages of between 6.3% and 31.7%, depending upon drive cycle, and that high efficiencies can be achieved at practical operating pressures.^[1]

TEG

Thermoelectric generator are another option to recover heat from the exhaust pipe to reduce vehicles fuel consumption.^[2]

Exhaust Heat Recovery on internal combustion engines with Rankine Cycle Systems

Passenger cars

Facing the new American, European, Japanese or Chinese regulation, more and more stringent concerning CO2 emissions, exhaust heat recovery sounds like one of the most efficient ways to recover a free energy, since heat is generated in many ways by the engine. Numerous companies develop system based upon a Rankine Cycle:

BMW

The German company has been one of the first major to study exhaust heat recovery with a Rankine system called Turbosteamer.^[3]

Honda

Honda also develops a module based on a Rankine Cycle to improve overall efficiency of hybrid vehicles, by recovering the heat of the engine and turning it into electricity for the battery pack. In the US highway cycle, the Rankine cycle system regenerated three times as much energy as the vehicle’s regenerative braking system.

Exoès

A French company, Exoès is specialized in designing and manufacturing exhaust heat recovery systems based on Rankine Cycles. The system EVE, Energy Via Exhaust, leads to fuel savings from 5 up to 15%.^[4]

Barber Nichols

Barber-Nichols Inc. develops Rankine technologies for vehicles.^[5]

FVV

The German consortium unites the majority of internal combustion engine manufacturers across the world. Two task forces are currently studying exhaust heat recovery systems on passenger cars.

Trucks

Renault Trucks: As a part of the All For Fuel Eco Initiative, Renault Trucks studies a Rankine system for long distance vehicles that could lead to 10% fuel savings.^[6] The goal is to produce enough energy to feed the components and electronic auxiliaries with electricity and reduce the fuel consumption by reducing the load on the alternator.^[7]

Trains

IFPEN, Enogia and Alstom are developing a system called Trenergy dedicated to improve train fuel efficiency.^[8]

Exhaust heat recovery in sport

Fuel efficiency, reduction of CO2 emissions, reliability and costs are necessary parts of Formula 1 manufacturers’ strategies. Automobile sport competitions is a good place to try and assess technologies that, once reliable, are adapted to private cars. As much as kinetic energy recovery system, Formula 1 constructors have produced one of the first exhaust heat recovery systems. Nowadays these devices are essential parts of embedded technologies on F1. Besides, heat recovery will be mandatory for the first time in 2014’s F1 Championship. Manufacturers, like Renault (ERS-H),^[9] test their systems, which also drives the turbo to improve its reactivity –and the torque at low regime- after braking.

References