Gasoline direct Injection (GDI) is set to take over the petrol engine world. Mitsubishi were the first of the current batch to bring one to market, although the first GDI engines were built over 70 years ago, but now every car company is working on one. GDI squirts directly into the combustion chamber thus avoiding the traditional puddle of fuel sloshing around the inlet port, giving much better fuel control, reducing wasted fuel and giving better emissions, CO2 and MPG.
But surprisingly it effectively raises the fuels’ octane rating. Because only air is admitted through the inlet valve (slightly more air than a PFI engine because the fuel is not taking up space, and so potentially more power) and heated up by compression, the fuel is not being heated. So we can run higher compression ratios without pinking and performance is significantly improved, its like running on 130 octane race fuel.
But there are down sides, the high pressure mechanical fuel pumps, up to 200bar, are similar to diesel pumps and can be a bit noisy. All the fuel must be injected during a fairly narrow crank angle, starting as late as possible on the compression stroke and higher revs need higher pressures to get all the fuel in before the spark, using off the shelf injectors can limit revs. Limited time for the fuel to mix with air leads to clumping and higher smoke/soot generation, affecting oil life and engine wear.
But the interesting twist is that if the intake air is hot enough, you can run the engine as a diesel during cruse conditions and get greater efficiency by avoiding throttle losses. Expect duel mode spark ignition / diesel engines in the next five years.