Monday, 4 April 2016

The BRM H16 engine – part 4: The quest for reliability and conclusions

It is fair to say that the development of the BRM H16 suffered due to numerous reliability issues. Vibration and resonance issue was by far the biggest cause of these issues.

First engine test bed runs resulted in broken inlet camshafts at the shoulder for the driving gear. Initially this failure was attributed to the high loads in the camshaft. In order to try to cure the problem, a temporary additional camshaft bearing was added for both inlet camshaft drive gears to prevent the gear being over-hung. With this arrangement the engine could be revved up to approximately 6000rpm, when it started to vibrate violently, especially at the fuel injection units.

Further investigation by BRM highlighted that the 0.75” diameter torsion shafts that drove the output gears were bright blue in their centres. This unusual observation confirmed that the natural frequency of the torsion shaft coincided with certain modes of vibration of the whole engine. A torsion shaft with a diameter of 1.15” was required to increase its natural frequency above the exciting range. Unfortunately packing space would not allow such large shaft diameters; instead the maximum diameter allowed was 0.95” which allowed the engine to rev up to 10,000rpm for short periods.
Output gear pack with steel 'spectacle' plate

With this running, failure of the gears or their bearings that connected the two crankshafts would then occur. In order to cure this issue the diametrical pitch of the gears was changed (from 8 d.p. to 6 d.p.) as were the arrangement of the bearings to increase stiffness (by four times). This redesign would then allow an engine to run for up to 4 hours; just an adequate duration for racing. With this design re-work the torsion shafts were now further increased in diameter to 1.05”. The shafts were also made of maraging steel to try to increase their working life. The next difficulty came due to the bearings for the centre gear which connecting the output shafts working loose in their magnesium case within about 2 hours. The bearing location was therefore reinforced with steel ‘spectacle’ plates. By now, it was however apparent that the vibration would need curing at the source.

A pair of eight-pin type crankshafts were manufactured. The extra balance weights of this crankshaft caused the offset between cylinders to increase by 0.4” to 1.05”. This in-turn meant new crankcase castings were required. It was also necessary to increase the crankpin diameter to 1.875” from 1.5”. Since this modification would take considerable time due to re-tooling, the decision was made to prove off the theory by welding and bolting steel rings onto the balance weights of the original design crankshaft. It was possible to add only four additional weights could be added to each crankshaft, but as each pair weighed 2lbs, there was still a substantial increase in inertia. This crude modification proved successful to the extent that the engine could now be used in a car. In fact for the majority of the 1966 racing season the H16 ran in this form; even winning the US Grand Prix in a Lotus.

The downside of these modifications was a weight increase of the engine, which now tipped the scales at 520lbs. When the mk2 engines were available (with correct 8 pin cranks), problems with connecting rod failures resulted in destruction of all the engines. The connecting rod failures were attributed to a machining mistake which caused a stress concentration point in the con-rod bolt bore.

BRM kept experimenting with the engine, for example chrome plated steel and then chrome plated aluminium cylinders. The latter items were reasonably successful, however an increase in oil consumption was noted with their use. Camshafts and tappets by now were now made of case-hardened En-39 steel. This modification along with the new crankshafts allowed the removal of an additional steady bearing on the engine which reduced mass by 10lbs.

The biggest gain in engine reliability came as a result of BRM’s greater understanding of the working life of each component. BRM introduced a “ruthless scrapping” policy, for example; output gear bearings were changed every race, output gears, camshaft driving gear bolts and dowels every 10 hours. Although the engine reliability was now far improved, the vibration characteristics caused by it resulted in a spate of gearbox failures.

As well as the H16 being heavier than anticipated, low engine speed torque was below what was expected. For example at 7000 rpm the 1.5 litre V8 produced 140 hp, whereas the 3 litre H16 produced only 240 hp. The specific fuel consumption of the H16 was also high which further increased the required car mass due to higher fuel loads. In order to improve both these items, the intake ports were reduced in diameter by 0.1in and the valve lift to 0.35in. This resulted in 4-5% more power at 7500 rpm and a 3% reduction in fuel consumption. Conversion to upstream fuel injectors also gave a considerable improvement in power and fuel consumption at lower engine speeds.

So what could be concluded about the 2 valve 3 litre H16 engine. It was not the success that was hoped. This was due to initial reliability issues stalling development, an engine that was both heavier and with less low speed power performance than anticipated, an engine that required high levels maintenance and the final death-nail was that even when engine reliability was acceptable, gearbox reliability became a problem due to engine vibration.

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.