Automotive powertrain engineers, in Europe as well as North America, continue to invest in research aimed at bringing together the different merits of diesel and gasoline engine technology. Their goal is epitomised in the HCCI (homogeneous charge, compression ignition) principle, whereby the superior fuel efficiency of a diesel is maintained, but with fewer of its drawbacks, notably regarding weight and NOx (oxides of nitrogen) emissions.
More complete combustion is achieved in an HCCI engine, benefitting fuel economy and cutting particulate emissions. The start of combustion is ‘held back’ until the fuel and the air are thoroughly mixed and, crucially, because a regular diesel’s surge in combustion chamber pressure and temperature, at the start of injection, is avoided, so too are the temperature peaks which lead to a diesel’s ‘high NOx’ reputation.
Through commonisation of parts and manufacturing processes, Mazda will be well placed, by means of production line flexibility, to deal with fluctuations in relative demand for the two types of engine.
Few development engineers expect to see an HCCI engine in commercial production before 2020, such are the combustion control challenges, especially those which arise over part-load versus full-load operation. In some quarters it is felt that ‘part time’ spark ignition might be required. The question of an optimised fuel formulation – and its availability – is a further obstacle to be overcome.
Nevertheless, combustion researchers remain undeterred. At the same time, moves are being made in Japan, in the passenger car field, to bring gasoline and diesel combustion closer in a quite different way, albeit with eventual HCCI potential. Mazda has recently unveiled what it calls its SKYACTIV next-generation engine strategy. Gasoline and diesel variants were designed and developed together, in a combined programme aimed at delivering significantly greater fuel efficiency and lower emissions than the company’s current engine range.
Mazda has chosen a common compression ratio (CR) of 14:1 for both the gasoline and diesel variants of its new SKYACTIV engines, a CR that is unprecedentedly high for a spark-ignited engine and unprecedentedly low for a diesel.
Through commonisation of parts and manufacturing processes, Mazda will be well placed, by means of production line flexibility, to deal with fluctuations in relative demand for the two types of engine. Those could arise from changing price differentials between the fuels needed by spark-ignition (SI) and compression-ignition (CI) engines, conceivably tied in with the influence of emissions legislation in the coming decades, which could include SI natural gas engines displacing diesel power in some markets.
Adoption of direct-injection for the SI versions clearly makes them more ‘diesel-like’ in fuelling and combustion management. More startlingly, Mazda has chosen a common compression ratio (CR) of 14:1 for both the gasoline and diesel variants of its new SKYACTIV engines, a CR that is unprecedentedly high for a spark-ignited engine and unprecedentedly low for a diesel.
This new ‘marriage’ that Mazda has arranged between gasoline and diesel must be taken seriously.
It implies similar maximum cylinder pressures for both types, nullifying the normal expectation that a diesel’s cylinder block and head, and its crankshaft and bearings, must be stronger and therefore heavier – or of higher-grade materials – than a gasoline equivalent’s. To achieve such CR commonality, Mazda is having to push the technological boundaries in new directions. Gasoline-fuelled SKYACTIV-G engine models feature electronically-controlled variable valve timing, which is employed effectively to lower the CR under part-load conditions. Meanwhile the SKYACTIV-D diesel’s Denso common-rail fuel system can muster up to nine injection events per cycle – part of a strategy needed to ensure cold starting and stable combustion during warm-up.
Mazda’s powertrain engineers have a reputation for ploughing their own furrow, without being diverted by technological conventions, the most notable example being their refusal, in the face of critics and sceptics, to give up on the rotary engine. So this new ‘marriage’ that Mazda has arranged between gasoline and diesel must be taken seriously.
The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.
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