Unforeseen consequence

Sam Adham, Senior Powertrain Analyst

19 November 2018

gasoline fuel efficiency

Diesel and gasoline share a common factor in that there is a delicate trade-off to be made between emissions compliance and added cost. For gasoline, however, there is an additional dimension: CO₂ reduction. The main research focus for gasoline engines these days is, unsurprisingly, fuel efficiency.

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.

There is a clear trend towards high fuel injection pressures. The vast majority of 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.

“In the drive towards diesel-like efficiency, that may come with diesel-like emissions and cost, if the wrong approach is taken”

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 RDE and particulate number limits has accelerated the development of gasoline particulate filters. Broad adoption is expected in Europe and China, although with a very minor uptake in North America. Incidentally, adding a GPF increases the exhaust back pressure, which compromises fuel efficiency. The 2018 Volkswagen up!, for example, has a 1.0L TSI engine with a coated GPF integrated in a 4-way catalyst and a 350 bar fuel injection system. In other words, a substantial added cost on an A-segment car where the margins are already small, which shows the lengths that manufacturers are going to in order to be compliant.

“There is a strong possibility that we will see SCR and LNT on gasoline for the first time”

It would seem that the consensus within the research community is that lean combustion is the next natural step for gasoline fuel efficiency. There are a couple of major approaches – namely stratified and homogenous – with varying degrees of efficiency benefits, and burdens on the aftertreatment as a consequence. Care must be taken, however, because in the drive towards diesel-like efficiency, that may come with diesel-like emissions and cost, if the wrong approach were to be taken. In general, running lean will generate more NOx emissions and further aftertreatment 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 SCR and LNT on gasoline for the first time.

Efficient and hybridised gasoline is supposed to be the combustion-based alternative as diesel declines. Taking into account the prospect of these added costs and complexities that have always been the mainstay of diesel, there is a risk that we will see the same pattern of stigmatisation and penalties occurring on gasoline, which would surely accelerate the demise of the internal combustion engine.

Acronyms
GPF = Gasoline Particulate Filter
LNT = Lean NOX Trap
RDE = Real Driving Emissions
SCR = Selective Catalytic Reduction