Most of us are quite clear about the advantages of modern diesel engines compared with their petrol counterparts. Similarly, the majority of people who know anything about motoring can cite the advantages that petrol engines have over diesels
They’ll tell you the spark-ignition engines are smoother, rev harder, are quieter, more powerful and cleaner. While everybody knows that diesels are more frugal and have better torque characteristics, they’re supposedly slower and noisier. These same people, though, would probably have difficulty imagining an engine which is neither a petrol nor a diesel, a motor that ditches most of the disadvantages of each, and capitalises on the merits of both.
A petrol engine with no spark plugs might sound quite unlikely, yet such a thing really exists – even if only in the laboratory – and it’s likely to be the power unit of choice when emissions legislators stamp down on petrol and diesel engines as we know them, perhaps prior to fuel-cell powerplants making their mark. Volkswagen is working on a new internal combustion engine of this type, one designed specifically to run on CO2- neutral synthetic fuels (i.e. fuels derived from vegetation) holding the potential for major reductions in particulate matter (PM), CO2 and oxides of nitrogen (NOx).
Known in Volks-speak as the Combined Combustion System (CCS), this fascinating development is both an evolution and a convergence of modern diesel-engine and petrolengine technologies, and its principle is more than a little similar to the Homogeneous Charge Compression Ignition (HCCI) concepts with which a number of engine makers and seats of learning have been experimenting for some years now. Vauxhall-Opel showed us a concept recently, in its Vectra.
In this it combines features of both spark-ignition and diesel engines, promising the high thermal efficiency of the diesel, but without the usually attendant high emissions of NOx and PM, and it can operate with a variety of different fuels. In the HCCI concept, fuel is pre-mixed homogeneously with air, as it is in a conventional spark-ignited petrol engine, although in this case using a considerably higher proportion of air to fuel. When the piston reaches top dead centre, this lean fuel vapour auto-ignites due to the heat of compression – just as it occurs in a diesel engine.
Of course, were you to rely on such a system to initiate combustion in a conventional petrol engine, you would end up with the mixture detonating randomly (engine knock or pinking), and this would create undue noise, rough running, a loss of efficiency, and ultimately engine damage. But in an HCCI engine, due to the high air-to-fuel ratio, this knock doesn’t cause damage, because the excess air limits the temperature of the burned gases, so combustion is ultimately cooler. Once you have eliminated the possibility of engine damage, auto-ignition becomes a useful and desirable feature. The timing of this auto-ignition is crucial to the power production and clean running of the engine, yet two means of controlling combustion timing (the start of injection in a diesel, or spark ignition in the petrol engine) are dispensed with. Also, the rate of combustion heat release is not controlled either by the duration or the rate of fuel injection (as in the diesel engine) or by the combustion-flame propagation time (with the petrol engine).
Yet it can be controlled; HCCI research has shown that by knowing the precise specification of the fuel being used, along with its density and temperature, it is possible to accurately predict the start of combustion. The trouble is that fuels are generally not produced to sufficiently tight a specification, and it is this problem of timing-control difficulty that presently keeps the convergent engine at an arm’s length from the motorist. Nevertheless, the boffins are hopeful. So, as is not the case with the petrol and diesel engines of today, HCCI combustion takes place spontaneously and homogeneously without flame propagation, and this eliminates areas of heterogeneous (unmixed) air/fuel mixture – the bugbear of current internal combustion engines. Additionally, HCCI is a lean combustion process, which produces a lower local flame temperature, which in turn means NOx emissions are reduced. With partial homogeneous-charge diesel systems possibly ahead on the market, and auto-ignited petrol systems also likely to appear, Volkswagen sees the potential to evolve those systems into one novel powertrain running on a single fuel. The CCS system can burn synthetic biomass gasto- liquid (GTL) fuels similar to naphtha or paraffin, but also conventional diesel or petrol, and if conventional diesel is used there is an immediate reduction of NOx and PM emissions – in the order of 50 per cent. Burning a synthetic GTL fuel similar to paraffin shows even greater potential though, with reductions of around 80 per cent, giving completely smoke-free combustion. The use of a synthetic biomass fuel could theoretically even create a combustion process that is totally free of PM and NOx emissions. All the while, don’t forget, that unique diesel advantage of high thermal efficiency is retained.
Volkswagen is refining the specifications of what it sees as the ideal fuel for use with CCS, and sees the distinct possibility of a fuel which could be supplied without problem, using the existing diesel supply infrastructure. Coincidentally, leaders in HCCI research and development at the University of California, Berkeley, have run a project engine based on a 1995 VW 1.9 TDI unit suitably converted to HCCI operation. It has been tested with various fuels, including propane, butane and methane, and all have produced very encouraging results.
It is clear that vehicles powered by the internal combustion engine will be with us for a long time to come, probably until fuel cells have made production and have proved themselves, and in this respect we may well be looking at the year 2025. It really is no accident that so many industry and academic organisations – even the US Department of Energy – are investing heavily in HCCI research, as this clean, comparatively simple and undoubtedly fuel-efficient concept has the capacity to stretch out dwindling crudeoil reserves.
Once fuel specifications are suitably tightened, and a way is found to control auto-ignition with complete accuracy, this convergent engine technology could well become the next, and possibly the ultimate combustion engine before the long-awaited advent of the fuel cell.