Wednesday, 9 March 2016

The BRM H16 engine – part 2: Engine layout

The H16 was a clear descendent of BRM’s 1.5 litre V8 engine. The H16 utilised the same valves, springs, tappets and cam forms. It was originally planned that the H16 would use the same pistons, liners and connecting rods as the V8.

BRM H16 layout

The crankcase of the H16 was a spacious unit based on the practices of the earlier V16 unit in order to prevent oil churning issues.

An essential feature of the H16 engine was to employ a central inlet port between the pairs of camshafts, as utilised on the later form of BRM’s 1.5 litre V8. It was also intended to use a common inlet camshaft (on each engine side); this could be achieved by reversing the offset of the pairs of connecting rods on the upper and lower crankshafts. There would be no issues with the exhaust ports as they could exit at the top and bottom of the engine. In practice BRM realised that this arrangement brought each crankshaft too close together, which could have been solved by widening the included angle between the valves, however this latter item would have been undesirable for combustion and as such the single inlet camshaft design was deemed not acceptable. Instead twin inlet camshafts were utilised on each side of the engine, which meant the valve included angle could be reduced. The penalty of this was an increase in engine bore size, weight, bulk and centre of gravity.
BRM H16 layout
As mentioned previously, the H16 used the combustion chamber from the 1.5 litre V8, but with a slightly narrower included valve angle and a tangential inlet port. With the narrower included valve engine on the H16, it became apparent that there was not enough space for the upstream fuel injection system which the V8 utilised. Previous research by BRM had shown that upstream fuel injection resulted in better specific fuel consumption and a wider range of mixture tolerance. The H16 was therefore to use downstream fuel injection which was anticipated would result in a slight performance loss and a poorer idle.

BRM swirl rig
BRM’s work on the V8 engine had showed that some port induced swirl was beneficial to ensure a more homogeneous charge, however a penthouse piston crown tended to kill this swirl motion as it entered the cylinder. Flow tests showed some disadvantages of high levels of swirl in the intake port; for example lower cylinder filling (port flow) and also in extreme cases to centrifuge heavy fuel onto the cylinder walls.

A method to improve mixture homogeneity on the H16 was therefore sought. The solution settled on was to give the stainless steel tube supplying to the central injection nozzle an aerofoil section and a 2° angle of incidence. Flow tests were conducted to help determine where this aerofoil should be positioned radially relative to the inlet valve stem. This was done to try to send charge around the back of the stem to reduce flow losses due to the valve stem. BRM’s flow rig showed a gain in airflow from 112 to 115 ft3/min.

BRM H16 induction trumpet showing aerofoil section fuel feed tube

In order to promote good coolant flow from lower to upper cylinder head, prevent leaks and also increase stiffness, the pair of heads on each side was cast in one. This made for a very complex casting and the resulting foundry issues caused a 6 month delay to the project.

Cast iron cylinder liners were utilised on the H16. During BRM’s earlier V8 development it was noted that cast iron liners as well as costing a quarter of a steel equivalent, also resulted in a slight increase in power. The disadvantage of cast iron liners noted by BRM was that any valve to piston contact experienced during development testing also tended to result in a shattered liner which led to total engine destruction due to the uncontrolled piston. With steel liners valve to piston contact typically only resulted in a bent valve. This was especially problematic on the H16 as on early engines torsional vibration issues were noted which could produce dramatic changes in valve timing. These valve timing variations could then lead to the total destruction of an engine, which made it very difficult to determine the initial cause of the torsional vibration.

Each cylinder on the H16 had a bore and stroke of 69.85 and 48.89 mm respectively. The crankcase was vertically split with the split being offset 2.5 inches from the engine centreline. The crankcase was cast in LM8. At the lower front of the crankcase, a triple-gear scavenge oil pump was installed. A ribbed magnesium sump was utilised.

The connecting rods for the H16 used BRM’s existing V8 forgings. The crankshaft was a nitrided fully balanced affair in EN40 using 2.25 inch Vandervell VP1 main bearings and 1.5 inch diameter crankpins. The rear journal of the crankshaft contained serrations which were used to drive a torsion shaft which passed through the output gear to serrations at the end of the gear which were carried in a pair of roller and ball bearings. The forward end of the crankshaft drove a disc carrying four lobes which triggered the Lucas transistor ignition system. The shaft driving this disc also carried a skew gear which drove a pair of distributors at half engine speed.

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