Since the early 1990s, progressive limits on emissions of diesel engines have been the source of major technical challenges. Although these have been less demanding for off-highway vehicles than some automotive sectors, they have nevertheless required profound developments in engine and powertrain technology, notably exhaust aftertreatments, exhaust gas recirculation (EGR), and electronically controlled fuel injection. The price that is paid for this is greater fuel consumption, to compensate for the higher exhaust back pressure and reduced combustion efficiency that these measures bring with them. In the future, says Concepts NREC, significant savings in fuel economy will be required to address concerns of oil dependency and atmospheric CO2, while maintaining or improving upon current emissions levels.
Turbocharging has been and will continue to be a key technology development in this area. Measures such as EGR reduce the combustion flame temperature to limit the formation of NOx, but in doing so, reduce the oxygen available in the cylinder for combustion and the power output. Power levels must be restored by boosting the pressure of the charge using a turbocharger compressor.
At the same time, reductions in exhaust gas temperature mean there is less exhaust energy available to the turbocharger turbine, with a consequent emphasis on turbocharger efficiency. Novel combustion technologies that are being explored to meet the twin goals of emissions and fuel economy, such as homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI), and low temperature combustion (LTC), all exacerbate the problems for the turbocharger, demanding ever-increasing levels of boost pressure while reducing the energy available to the turbocharger.
Although the turbocharger has become an essential part of the diesel powertrain, it is not ideally matched to the engine. It is a rotordynamic, continuous flow device, unlike the engine, which has volumetric, intermittent flow characteristics. The operating range of the turbocharger is limited by the stability of the compression system, which if not monitored and controlled will surge and cause damage to itself at low engine conditions. Range is important in vehicles that must operate to the limits of engine speed and load conditions.
The turbocharger is a mature and highly refined device that achieves the right compromise between several competing technological and economic requirements. Efficiency, boost pressure, and range are all important goals, but so too are low inertia to combat turbo lag, and low cost for commercial success.
Durability and service life are very important for off-highway vehicles that may operate over extreme load cycles and in situations that are difficult to predict, such as very dusty conditions. In some circumstances, foreign object damage and blade fatigue may be life-limiting for the turbocharger. The achievement of the turbocharger manufacturers has been in getting the right balance of these factors rather than maximizing any single parameter, and designers know that further gains in one area will only come at a cost in others.
The simple turbocharger is reaching its limit in technical development, and any additional gains will be incremental, not fundamental. Future developments must come in the form of more complex turbocharging systems. The exhaust wastegate to bypass a controlled quantity of gas around the turbine was originally introduced to improve the match between the turbocharger and the engine over a range of engine speed and load conditions as demanded by users. The variable geometry turbine can be considered as a more sophisticated version of this, varying the size of the turbine to match the changing exhaust gas flow rate. In doing so, integrated with the engine management unit, it can provide additional control over the engine, for example, in setting the EGR quantity and in providing additional boost temporarily during engine accelerations.
Series turbocharging uses two turbochargers in series and is already in widespread use in heavy-duty diesel engines, but increasingly it is penetrating the light-duty sector. Higher boost pressures can be achieved with series turbochargers. It is technically possible to achieve comparable boost pressures with a single turbocharger, but the impacts on turbocharger efficiency and range are considerable. Even so, series turbocharging alone does not meet the requirements of most applications and drive cycles, and one or both turbocharger turbines are fitted with wastegates or variable geometry.
Current developments also include compressor bypasses, which allow one stage to be switched out entirely, and combinations of series and parallel turbochargers. For smaller vehicles, packaging a series turbocharger is a considerable problem, and integrated series turbochargers have been developed to address this problem. A compromise solution is to drive two compressor stages with a single turbine stage, gaining packaging advantage at the cost of some operating flexibility.
Recent developments of small, high-speed electric machines and solid-state power switching opens up many possibilities for electrically assisted turbocharging and e-boosting. These machines are similar in size and rotate at the same order of speeds, as turbocharger compressors and turbines, so compact integration of rotordynamic and electrical machines is possible.
The electric machine acts as a motor to provide additional power and boost at low engine conditions and as a generator when there is excess power at high engine conditions. An e-booster is an auxiliary air compressor, in series with the turbocharger compressor, driven by an electric motor, which can be switched in at times when extra boost is required.
Critical to these developments will be the onboard storage of electrical energy. Even the best batteries today have an energy storage density that is many times lower than that of hydrocarbon fuels. Such systems stand a better chance of succeeding if hybrid diesel-electric or gasoline-electric powertrains become widely adopted.
Nick Baines, Concepts NREC, wrote this article for SAE Off-Highway Engineering.