In its new report on downsized and boosted gasoline engine technologies (earlier post) the ICCT highlighted Ricardo’s work on an “extreme” Miller cycle concept with high (13:1) compression ratio (CR) and central direct injection in a downsized boosted engine.
The basic architecture of “Magma” concept is a boosted four-stroke gasoline engine with four valves per cylinder. The Magma concept architecture closes the inlet valve early to reduce pumping losses. A mechanical supercharger and fixed-geometry turbocharger fill cylinders effectively. A long exhaust stroke then extracts maximum efficiency.
There is no component in the engine which isn’t proven in production already, said Trevor Downes, chief engineer, engines, at Ricardo Innovations.
Under most conditions, Magma closes the intake valve at least 30 degrees before bottom dead center (BDC). The in-cylinder gases are thus expanded and cooled as the piston descends, before being re-compressed as the piston rises again. The compressed gases in Magma are lower in temperature than in a conventional engine as the spark plug fires, thereby helping reduce knocking and potentially allowing more advanced ignition timing for better efficiency.
The expansion ratio on the exhaust stroke is much higher than the effective compression ratio; the longer expansion provides extra power to the crankshaft and extracts more energy from the combusted fuel. There is also less work expended during the pre-combustion compression stroke.
Magma’s valve timing enables the geometric compression ratio to be pushed up to 13:1 but keeps the effective compression ratio to around 9.5:1. All this benefits thermodynamic efficiency.
The concept places particular demands on the boosting system. Early closure of the inlet valves leaves a smaller window for the charge fill the cylinder. Magma, which uses a mechanical supercharger and a turbocharger as well as two stages of intercooling to ensure the charge air is as cold and dense as possible as it enters the cylinder, runs on higher boost pressures than have been the norm.
In an article in Ricardo Quarterly Review, Trevor Downes explained that Magma compounds the pressure ratios across both boosting devices all the way across the full-load speed range and in the steady state, allowing for effective interstage cooling.
The ICCT report stated that the Magma engine shows a full-load ISFC benefit of nearly 8% compared with the baseline. Testing also found an 18% reduction in fuel consumption at the low speed light load condition of 2000 rpm/2 bar BMEP and an 8% reduction at a moderate load of 8 bar, over the baseline engine.
Ricardo continues to develop the concept.
C Rouaud, R Osborne, K Pendlebury, J Stokes, J Dalby (2015) “The Magma Engine concept - A downsized turbocharged gasoline engine with high compression ratio” 2015 JSAE Annual Congress (Spring)
Ken Pendlebury, Richard Osborne, Trevor Downes, Simon O’Brien (2016) “Development of the Magma Combustion System” 2016 JSAE Annual Congress (Spring)
Tony Lewin “Transformative Technology”, Ricardo Quarterly Review Q3 2016