Saturday, 2 April 2016

The BRM H16 engine – part 3: Performance and development

Given that the H16 utilised effectively 16 cylinders from their 1.5 litre V8 engine, BRM had anticipated that the H16 would produce approximately twice the power of their 1.5 litre V8 (i.e. 420 to 440 BHP). In practice this was not the case. Many of the initial investigations into this related to the suspected increase in oil churning and mechanical friction. In practice after analysing the heat rejection to both the oil and coolant, BRM concluded that this could not be the full cause. During this work BRM did note that early engines tended to overheat due to water flow stalling. A redesigned water pump impeller cured this issue and a smaller water pump also reduced friction and caused an unexpectedly large increase in engine power. Based on the last observation, BRM concluded that the combustion chambers had previously being over-cooled.
BRM H16 power curves compared to 1.5 litre V8

The volumetric efficiency of the H16 was very good. By using fuel flow and exhaust air-to-fuel ratio measurements, the volumetric efficiency of the H16 was determined to be 104% at 8000 and 9000 rpm, rising to 110% at 10,000 rpm. These measurements were confirmed by compression pressures, which showed the engine’s breathing performance was better than BRM’s 1.5 litre V8. This was attributed to the narrower valve angle and tangential inlet ports.

Initial running on the H16 highlighted a variation in cylinder-to-cylinder air to fuel ratio (AFR) of 5% with ideal mixtures and up to 10% with richer mixtures. As a result the cylinder-to-cylinder combustion speed varied by up to 10%. Following the calibration of the fuel injection equipment, the cylinder-to-cylinder AFR imbalance reduced to 0.5%.

The H16 used BRM’s ‘three-hemisphere’ combustion chamber that had been developed on the later 1.5 litre V8. On the V8, this combustion chamber had caused a 15-20% increase in combustion speed above 10,500rpm. This in turn caused engine power to keep increasing with engine speed up to 11,750 rpm.

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