Diesel and gasoline share a common problem because there is a tricky trade-off to be made between emissions compliance and added cost. For gasoline, however, there is an additional dimension: CO₂ reduction. The main research on gasoline engines these days is, unsurprisingly, fuel efficiency, which directly correlates with CO2.
The gap with diesel is closing, but in more ways than just fuel efficiency. Expensive hardware, lean combustion, high pressure injection, compression-ignition – all quintessential characteristics of diesel engines – are now appearing on gasoline engines.
Efficient and hybridized gasoline should be the combustion-based alternative as diesel declines. Considering these added costs and complexities that have always been part of diesel, there is an unknown risk that the same pattern of reputational blight and penalties will occur on gasoline engines. This could/would accelerate the demise of the internal combustion engine while promoting electric vehicles.
There is a trend towards high fuel injection pressures. Most OEMs worldwide have plans to introduce 350 bar fuel injection systems in the next few years, either across their entire portfolio or selected vehicles, and those decisions are taken for all production models, independent of the market. There are already investigations being made into 500 bar in the long term.
Raise the Bar
- 200 bar fuel pressure produces many large particulates
- 350 bar fuel pressure reduces the exhaust particulate number by 70%
- 500 bar fuel pressure further reduces particulate number by 50% versus 350 bar
However, diesel-like efficiency, that may come with diesel-like emissions and cost, if the wrong approach is taken,” says Mary Gustanski, Chief Technology officer of Delphi.
As far as pollutant emissions are concerned, the current focus is on particulate number reduction, but NOx could also come under intense scrutiny with the trend towards lean gasoline combustion. The introduction of Real Driving Emissions (RDE) and particulate number limits has hastened the development of gasoline particulate filters. Significant adoption is expected in Europe and China, but a minor uptake in North America.
Incidentally, adding a Gasoline Particulate Filter (GPF ) increases the exhaust back pressure, which compromises fuel efficiency. The 2018 Volkswagen up!, for example, has a 1.0-liter TSI engine with a coated GPF integrated in a 4-way catalyst and a 350-bar fuel injection system. This is a substantial added cost on an A-segment car where the margins are already thin, which shows the lengths that manufacturers are going to in order to be compliant.
“There is a strong possibility that we will see Selective Catalytic Reduction (SCR) and Lean NOX Traps (LNT) on gasoline for the first time,” says Gustanski.
The consensus within the research community is that lean combustion is the next natural step for gasoline fuel efficiency. There are a couple of approaches – namely stratified and homogenous – with varying degrees of efficiency benefits, and burdens on the after-treatment consequently.
Running lean will generate more NOx emissions and further after-treatment options will need to be considered, not least because performance of current 3-way catalysts is limited at lean conditions, but there is a strong possibility that we will see Selective Catalytic Reduction (SCR) and Lean NOX Traps (LNT) on gasoline engines for the first time.
- GPF = Gasoline Particulate Filter
- LNT = Lean NOX Trap
- RDE = Real Driving Emissions
- SCR = Selective Catalytic Reduction