Issue #229 Crunching Numbers | Combustion, Volume, Compression, Chamber, Engine

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Issue #229 Crunching Numbers

Written by Robert Smith
Wednesday, 25 April 2007
Choosing the compression ratio for an internal combustion engine is a bit like Goldilocks trying out the Three Bears’ chairs. Ideally, the CR should be just right: as high as possible (for maximum efficiency) within the limitations of fuel, mechanical components and combustion chamber design. Too small, and you’re leaving horsepower on the table; too large, and the result will be mixture detonation leading to engine damage. I got a call recently to look at a Norton Atlas that had symptoms of too high compression: it was really hard to kick over, and when the engine started, it detonated badly even under no-load conditions. Time to find out what was inside. The engine was from 1968 when Norton introduced a high-compression head for the Atlas. But the motor had also been fitted with later higher compression Commando pistons. The shop owner had correctly diagnosed the problem and fitted a compression plate under the barrels. But how would we know if the compression plate was thick enough to get the desired compression ratio? To calculate this, we needed two pieces of information: the swept volume of the cylinder, and the volume of the combustion chamber. Combustion chamber volume is usually measured by clamping a sheet of Plexiglas over the cylinder head and using a suitable fluid (engine oil works) to fill the combustion chamber, measuring the amount used. For a stock 750 Commando, the numbers work out something like this: Swept volume = 372.5 cm3 Combustion chamber volume ~47cm3 Compression ratio = SV + CCV CCV (SV = Swept Volume) (CCV = Combustion Chamber Volume) So for our stock Commando engine (373.5cm3 per cylinder), 372.5 + 47/47 = 8.9:1 (This ignores the thickness of the head gasket and the valve pockets in the pistons.) To create the 1972 Combat engine, Norton skimmed one millimetre from the head to increase the compression ratio. Let’s assume the combustion chamber was cylindrical where the material was removed. We can calculate the change in volume using the equation for the volume of a cylinder v = πr2h. The new combustion chamber volume would therefore be approximately 41.4cm3. So the Combat’s compression ratio would be: 372.5 + 41.4 = 10.0:1 41.4 This is the figure usually quoted for the Combat Commando, so our calculation seemed to work. We took a good look at the high compression head fitted on the Atlas and measured the depth of the combustion chamber, finding it to be approximately 1.25mm shallower than a stock Commando Combat head. Again, assuming the combustion chamber was cylindrical where the material was removed gave us a combustion chamber volume of 35.9cm3. We also needed to take into consideration that the Atlas had been bored to +0.060”. That increased the swept volume to 388cm3. So, for our badly detonating Atlas, the estimated compression ratio worked out like this: 388.0 + 35.9 = 11.8:1 35.9 The classic Norton hemi combustion chamber wasn’t designed for compression ratios that high: modern chamber design with narrow valve angles and large squish bands allow CRs or 13:1 and more, but in a Norton you’d need Avgas or race fuel. The solution was to fit a compression plate—but how thick? Let’s say our target compression was 8.0:1. We could calculate the required extra volume by using our formula backwards. To find the new volume we needed for the combustion chamber: If: 388 + x = 8.0 x Then: x = 388 =55.4cm3 8.0 – 1 So we needed to increase the combustion chamber volume by: 55.4 – 35.9 = 19.5cm3. At a diameter of 74.5mm (the stock Atlas bore of 73mm plus 0.060” or 1.5mm), the required height from v = πr2h works out like this. h = v = 19.5 = 0.44cm (4.4mm) r2 x(3.725)2 The shop owner had cut the compression plate from 3/16” aluminum. This is slightly thicker than the 4.4mm required, and we estimate the Atlas’s new CR at around 7.8:1. Should be just right! Comments (0) Subscribe to this comment's feed Write comment You must be logged in to a comment. Please register if you do not have an account yet.