RaerDesign

Engine Technology

At present the four-stroke engine is replacing the two-stroke for racing application. What developments are now being carried out on the racing two-stroke mainly concern higher operating speed to produce more power without ever applying new technology. Maximum power is now produced at engine revs above 13,000RPM, whereas, in the not to distant past, it was at 12,000RPM. It is less safe for the rider to have higher and higher revving engines due to traction issues. In the past, companies such as Yamaha were innovative with ideas such as power-valves and variable compression cylinder heads, but now very little in the way of new design technology finds it way onto Grand Prix two-stroke engines. Cleaner burning two-stroke engines could see a revival of this engine. One manufacturer who attempted this but failed to succeed was Bimota. Here is an insight into what could have been…

Bimota V Due – History.

The Bimota V Due 500ei was built as a Grand Prix racer back in the early 1990s. Due to design faults within the engine this ambition had to be set aside in favour of turning it into a sports road bike. This change in direction was solely for commercial reasons. Only, the inherent problems prevailed with the change of application. The engine was, and remains, the most advanced automotive production engine ever produced. At that time the car industry was considering the two-stroke engine as an alternative to the four-stroke, therefore kept a watchful eye on Bimota’s progress.
The two-stroke is a more efficient engine; also it has the potential for reduced exhaust emissions.
(Ford invested heavily in this alternative technology at the same period of time with the three cylinder Orbital engine.)

The Bimota V Due was an intelligent engine. Instead of having 2D maps for the ignition and fuel injection, Bimota had a continually changing circuit that altered the program every time the engine fired. This was supposed to correct the running of the engine and provide optimum performance. What the small but ambitious company had done in its quest for Grand Prix glory was produce an engine with design faults that were exacerbated by futuristic technology, a situation that was not known to them at the initial conception of their project.

Design Faults.

1. The Bimota V Due broke the first rule of two-stroke engine design by having a downward pointing front cylinder.

The simplicity of the two-stroke engine can be deceiving, where by having a lower cylinder that never adhered to the ‘chimney effect’, which is paramount to performance and reliability, they were never going to be successful. To explain the benefits of an upright cylinder and a downward exiting exhaust, with the header pipe (the first section of the exhaust as it leaves the cylinder).

Such an arrangement goes against the natural inclination of hot expanding gas, which has it leave the cylinder in a more controlled and uniformed manner. In basic terms, it forces itself out without wanting to go in that manner, therefore has a consistency to it that gas leaving of its own accord would never have. By having the ideal set up, high pressure is maintained at the mouth of the exhaust port and the sonic pulse will be more uniformed with less interruptions giving an overall more consistent performance. The sonic pulse is the greatest influencing factor on power at high revs in the racing two-stroke engine. There is the secondary benefit of not having fresh charge follow the exhaust fumes out of the exhaust port if there is a reluctance for gas to flow in this direction. For both these reasons this is why so may two-stroke engines experience difficulties with the bottom cylinders when the engine is in a V configuration. The bottom cylinder will always produce less horsepower than the top cylinder when placed in a V at the front of the engine.
Bimota further exacerbated the situation by means of their direct injection that was squirting raw fuel into the cylinder, which was landing on the spark plug in the bottom cylinder, causing plugs to foul. Cold wet fuel hitting the plug at high operating temperature is destined to crack the ceramic; likewise un-atomised fuel can bridge the spark plug electrode gap.
(When fuel contained lead, it was more likely to short out spark plugs; thankfully this is now in the past.)
The loss of fuel straight out the exhaust port was held to a minimum because the injectors only fired as the exhaust port was closing by means of the rising piston. To quickly atomise the fuel, Bimota aimed the injectors to fire at the fast rising piston, hitting it and dispersing the fuel automatically into tiny droplets. Such an ingenious answer saved them from having the complexity and weight of a compressor for a high-pressure injection system. Air alone was supplied to the cylinder from the crankcases by means of the transfer ports with the mixing of fuel within a short period of time thereafter – a quarter of a turn of the crankshaft. Had they adhered to the ‘chimney effect’, the possibility of being successful at their first attempt would have been greatly enhanced.
(When Honda redesigned their RS125 completely, from what had been a moto-cross derived engine, they had an upright cylinder that was canted forward and a header pipe that exited directly downwards. For a racing reed-valve two-stroke engine this was the perfect set up to take advantage of the ‘chimney effect’ and run a relatively straight exhaust afterwards.)
Once informed of this design fault, what did Bimota do?
Bimota had originally run a single program to manage both cylinders, which on an intelligent engine was most definitely a mistake but rectified the situation by reverting to individual programs per cylinder. This marginally helped, as did their work on the injectors, but neither could overcome the fundamental flaw. What they needed to do was simply rotate the engine in the frame until both cylinders were 45 degrees from the upright, with exhausts exiting directly downwards - one in front of the engine and one behind. At the time Bimota were struggling financially, therefore the expense of a new chassis and modified gearbox casings, in relation to oil level and position, was too much for them.

2. The intention of Bimota, with the V Due, was to build a twin cylinder Grand Prix racer capable of challenging the four-cylinder opposition. To achieve this, and they did have horsepower figures that could do so, meant producing a short stroke engine more capable of higher revs than a long stroke variation. Conventional wisdom dictated this, but Bimota’s new technology did not favour this principle. A short stroke engine gave less time (in milliseconds) to mix the fuel with the air inside the cylinder. There was also less space available on the cylinder walls for the injectors (height wise). Such an engine has a slower piston speed at mid-stroke for any given engine speed, when, in this case, the fuel was fired at the rising piston. This meant there was less impact, reduced atomisation, of the fuel. Finally, had they went for a long stroke engine, this has a smaller bore which meant less risk of detonation because the fuel with the air are contained in a smaller circumference further enhancing mixture thereby reducing risk. A pocket of lean mixture could otherwise easily exist in a large cylinder bore, as was the case with the V Due. The known fact that the flame front has less distance to travel with a smaller diameter also applies.
Once again, Bimota could ill afford the expense of redesigning the engine once they knew this, therefore never changed the engine dimensions.

3. Bimota wrongly chose a reed valve induction that further complicated the problems. Reed petals are fixed shut only opening when the pressures either side of them are great enough to do so. When this happens depends upon their stiffness. On two-stroke engines, the exhaust pulse has a great influence on the next induction cycle. With an intelligent engine it is reading what is happening there and then, unaware the next engine cycle is dominated by what has went before it. If the engine fails to fire correctly on one cycle that failure registers with the next cycle but the engine has not read this therefore continues as if there is no problem, which in fact exaggerates the situation. To put this another way, the engine cannot predict the circumstances of its next cycle thereby wrongly assumes what fuel and ignition timing are sought. With a conventional reed valve and carburetted engine, a restricted amount of mixture will enter the crankcase if the engine has misfired on the previous cycle, but here if that were to happen, the air will be restricted yet the amount of fuel will not necessary be so, providing the wrong mixture. A conventional reed valve engine will quickly overcome any spluttering whereas the Bimota V Due would first make the problem worst before compensating. This also explains why riders complained the power band of this engine moved about in the engine speed range – the mapping could alter the power depending on the circumstances.
Bimota in the latter years of development, realised the reed valve was causing any rough running of the engine to prolong itself and started designing a new valve. What they were about to produce would only be of limited effect, whereas RaerDesign also had an induction valve that could rectify any misfire within one engine cycle. This would allow smoother running than that of any conventional reed or rotary disc two-stroke engine!
Bimota ran out of time and money before they could take advantage of what was being offered to them, with the company going into insolvency at that time.

Bimota were originally expert frame builders who took on this ambitious project without a solid base of two-stroke knowledge. The idea of an intelligent clean burning two-stroke was sound. They were able to overcome and improve some of the problems encountered, just like that entailed in any other development program, but were blighted by three rather simple looking flaws. The company solved any other minor problems associated with this engine adequately.