Abstract
Natural gas engines for commercial vehicle application are getting more attractive in view of operating cost and its potential to reduce greenhouse gas emissions. Different global emission regulations and market requirements did led to different technologies for gas engines. Still, the low price of natural gas and local incentives are the main drivers for fleet owners to invest into gas powered trucks, however, today only low compromises in view of transport efficiency are accepted by the market.
For spark ignited engines the lean burn concept worked well for EU III, EU IV and EU V emission compliance. The most stringent EU VI and US EPA14 emission requirements can be met well using stoichiometric combustion concepts combined with Exhaust Gas Recirculation (EGR) and three-way-catalyst systems. As discussed in this paper, there are challenges connected to that concept, like high thermal load on components, high knock sensitivity with low methane number fuels and a trade-off between high power density and thermal efficiency. EGR is one of the key enablers of a higher power density as it reduces the thermal load, suppresses the knock behavior and reduces part load throttle losses. EGR, though, is much more important than it is on Diesel engines.
Combustion system development focusses primarily on a short burn duration and optimum flame propagation. This can be achieved by promoting a high turbulent flow pattern throughout the combustion cycle by means of squish flow and swirl charge motion. CFD analysis shows, that both high and low swirl combustion systems can yield almost similar high thermal efficiencies. Yet, the level of squish flow and hence the type of piston bowl shape needs to be carefully matched to the charge motion.
One of the main drivers for a stoichiometric combustion concept is high conversion efficiency of NOX and CH4 with conventional three-way catalyst technology. For a heavy duty commercial vehicle application a long useful life target in combination with catalyst aging has to be considered in the aftertreatment concept definition, as catalyst aging processes are shifting the light-off towards significantly higher temperatures. Thermal management measures need to be applied to maintain a minimum exhaust gas temperature during the legal test cycle.
Apart from spark ignited engines, also dual fuel concepts with port fuel injection or high pressure direct injection (HPDI) are interesting alternatives. Port fuel injected dual fuel technology is seen to provide only limited potential to fulfil current and future most stringent emission requirements. HPDI technology promises high power density, diesel-like fuel efficiency and high Diesel substitution rates of up to 90%; however the entire HPDI system with its LNG tank modules including the high pressure pump is complex. Fleet owners are evaluating based on the total cost of ownership. Considering driving range, transport capacity, running cost and maintenance cost, a considerably shorter payback is seen for the SI engine concept.
KEYWORDS – Commercial vehicle, Natural gas engine, CNG, LNG, Dual Fuel, HPDI