Compression ratios in SI engines is limited because of the occurance of ignition knock. Does the introduction of electronic injection system has any effect in extending the maximum limit.
Think of a Diesel. Injection of fuel into the cylinder results in (nearly) immediate ignition. In Spark Ignition engines, the spark serves to ignite only if (a) the temperature is below the ignition temperature, or (b) the fuel isn't present in the cylinder (yet).
The maximum compression ratio in a spark ignition engines is 11
With various advanced valve timing strategies, such as the Atkinson cycle used in Toyota Prius engines, one should distinguish between the classic compression ratio (given by the piston stroke) and the effective compression ratio (given by the intake valve closing). In my opinion, the realistic ceiling is around 15:1 or 16:1, beyond which the benefits are negligible or even negative.
13:1 reported by Toyota in
http://papers.sae.org/2010-01-0174/
The new Mazda uses SKYACTIV Technology, of which cpmpression ratio is 14:1. It reports that its 3rd generation Mazda will be 18:1 with the same technology. To my knowledge, the compression ration should be the highest one in the current spark ignition engine.
The Mazda SkyActiv uses a variation of Atkinson Cycle with late intake valve closing. While the compression ratio measured based on geometry of the power assembly does yield 13:1 in N.A. and 14:1 in Europe, over much of its operation the actual compression ratio is much lower and the expansion ratio is higher (they are not equal in Atkinson Cycle operation). In practical terms, naturally-aspirated gasoline direct injection (GDI) engines in the U.S. are limited to about 11 or 11.5:1 compression ratio on 90-91 RON (87 aki) gasoline. Turbocharged GDI (e.g., 1.5 Ford EcoBoost) are limted to about 10.5:1 on the same fuel. PFI automotive engines are limited to about 10:1 - 10.5:1 on the same fuel. Use of cooled EGR can allow higher compression ratio and reduces enrichment at high BMEP.
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