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One for the motorheads
Because we motorheads know who we are...
The Waukesha CFR (Cooperative Fuel Research Committee) engine first came out in 1928. It is a variable compression ratio, single cylinder test engine. The current production model has a crankcase that debuted in 1948, hence it is the 48 model. The current production model has the same combustion chamber design as the first model, so a fuel tested on a 1930 version of this engine will give the same result as on a test engine built this year.
All of the test engines in use around the world are still built by the Waukesha division of Dresser, to my knowledge. I could be wrong.
There are different configurations or versions of the engine for the Motor method, Research method, and cetane measurements (diesel). If you want to get an AKI for a fuel you need two engines, a Motor version and a Research version. They are either 600 or 900 RPM engines, approx 600 cc, with four carburettors so fuels can be quickly changed. The test fuel is compared to a reference fuel. Knock is sensed a variety of ways. If a fuel knocks at the same compression ratio, etc, as a reference fuel, it will be labelled as having that AKI. That is why fuels can behave very differently in modern production engines, as modern engines have a wide variety of combustion chamber shapes, injection technologies, etc.
I used a similar engine during my university days, many years ago. I suspect there are a few other members on here who have run octane tests themselves. Please chime in.
A photo of an early model is attached. Current models are basically the same, but get more complicated instrumentation.
So, when BMW says that we should use 92 AKI, it is because the fuels they tested in the X5 engine, when tested on this Waukesha lab engine, had similar AKI ratings as reference fuels with an AKI of 92 (allowing a safety margin for local fuel production variation). My point is that since the ASTM test does not dictate how any particular engine will respond with any particular fuel (it only compares results on this engine, under a single set of conditions), your experience will vary.
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2007 X3 3.0si, 6 MT, Premium, White
Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue
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