The Six Stroke Engine

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The Beare Head offers an array of advantages, but it specifically offers a compact combustion chamber with a 50 per cent squish. Thus, the combustion in the center of the piston is concentrated, increasing the flame speed and the speed of combustion. In doing so the thermal stress on the piston is actually reduced.

An added benefit of this Beare Technology configuration is that it allows a higher bore stroke ratio, due to a lesser expansion of the piston. As there are no cut outs for valves, the crown of the piston can be slightly domed for higher strength and less weight. The 50 per cent squish offered by the Beare Dual Piston design, keeps the edges of the piston from being exposed to the flame. By doing so, it allows the use of a gapless L shaped compression ring to be implemented right to the top of the piston. Therefore ring flutter in the Beare Engine is reduced and almost eliminated.

The main source of hydrocarbon emissions is also reduced in the Beare Six Stroke Engine, the Crevices (these are narrow regions in the combustion chamber into which the flame cannot propagate because it is smaller than the quenching distance.) Crevices are generally located around the piston, head gasket, spark plug and valve seats, these crevices represent about 1 to 2% of the total clearance volume. The crevice around the piston is by far the largest; during compression the fuel/air mixture is forced into the crevice (density higher than cylinder gas since gas is cooler near walls) and released during expansion. The Beare Cylinder Head eliminates most of these pollution trapping Crevices.

Hydrocarbon emissions result from the presence of unburned fuel in the engine exhaust. However, some of the exhaust hydrocarbons are not found in the fuel, but are hydrocarbons derived from the fuel whose structure was altered do to chemical reaction that did not go to completion.

For example: acetaldehyde, formaldehyde, 1,3 butadiene, and benzene are all classified as toxic emissions. About 9% of the fuel supplied to the engine is not burned during the normal combustion phase of the expansion stroke. Only 2% ends up in the exhaust the rest is consumed during the other three strokes. As a consequence hydrocarbon emissions cause a decrease in the thermal efficiency, as well as being an air pollutant.

There are six principal mechanisms believed to be responsible for hydrocarbon emissions

Percentage fuel Escaping


normal combustion

% HC emissions




Oil layers






Liquid fuel



Flame quench



Exhaust valve leakage






Crevic and Piston Ring


Hydrocarbon Emission Sources

Oil layers - Since the piston ring is not 100% effective in preventing oil migration into the cylinder above the piston, oil layers exist within the combustion chamber. This oil layer traps fuel and releases it later during expansion.

Deposits - With continued use carbon deposits build up on the valves, cylinder and piston head. These deposits are porous with pore sizes smaller than the quenching distance so trapped fuel cannot burn. The fuel is released later during expansion.

Liquid fuel - For some fuel injection systems there is a possibility that liquid fuel is introduced into the cylinder past an open intake valve. The less volatile fuel constituents may not vaporize (especially during engine warm-up) and be absorbed by the crevices or carbon deposits.

Flame quenching - It has been shown that the flame does not burn completely to the internal surfaces, the flame extinguishes at a small but finite distance from the wall. Most of this gas eventually diffuses into the burned gas during expansion stroke.

When the exhaust valve opens the large rush of gas escaping the cylinder drags with it some of the hydrocarbons released from the crevices, oil layer and deposits. During the exhaust stroke the piston rolls the hydrocarbons distributed along the walls into a large vortex that ultimately becomes large enough that a portion of it is exhausted.


The Beare Head reduces wall wetting and carbon deposits.

Some thoughts on MotoGP

To help keep a cap on power and, hence, speed, the MSMA has decided to propose a reduction in engine capacity from 990cc to 900cc. "The intention is not to reduce performance but to prevent a continuous improvement in speed and lap times," according to the press release.

2004 to 2007 weight changes
2 Cylinders 135 Kg 133Kg - -2Kg
3 Cylinders 135Kg 140.5 Kg +5.5Kg
4 cylinders 145 Kg 148Kg + 3Kg
5 cylinders 145Kg 155.5 Kg +10.5 Kg
6 cylinders 155Kg 163 Kg +8Kg

The proposed changes to the rules also affect the minimum weight standards, adding more weight to engines with more than two cylinders from 2007.

The proposed changes above may indicate the technical direction that some manufacturers are pursuing for the future. As Honda is the most powerful voice among the companies, it is interesting that the proposed minimum weight for five-cylinder machines, such as the Honda RC211V (and Proton KRV5), has been increased the greatest amount. This may indicate that Big Red is already working on new engine configurations and is looking to abandon the V-5.

As two-cylinder bikes are the only ones to get a minimum weight decrease, might we see the introduction of a 900cc MotoGP V-Twin? If so, it wouldn't be as powerful, no doubt, but it would enjoy nearly a 50-pound weight advantage over a V-5-powered machine. And, as a Twin would have a 66-pound advantage over a six-cylinder-powered bike, it looks like the rumors of a Honda V-6 will not be fulfilled.

The MSMA is also looking at perhaps reducing the 2005 rule for a 22-liter fuel tank capacity (down 2 liters from current rules) for the 2007 season.

The introduction of 4-stroke machines to MotoGP has resulted in a huge amount of newfound interest in the class. Now, with revised regulations again on the table, the series might get even more interesting.

The Testastretta engine fitted to the Ducati 2002 998R has a bore of 104 mm. Unfortunately; such a large bore currently causes combustion problems with dramatically decreased efficiency. This stems fundamentally from the need to augment the injection advance and from the worsening of the "shape factor" of the combustion chamber which, with the reduction of the bore/stroke ratio, becomes ever broader and flatter. The "shape factor" is a critical synthetic value to check a combustion chamber's good operation, and a good indicator of its compactness and "thermal efficiency".

It should be borne in mind that aspirated racing engines require rather extreme valve lift and overlap angles; therefore, cavities are made in the piston crowns to prevent contact with the half-open valves. The combustion chamber is therefore practically contained in the piston cavities, such cavities becoming bigger as the stroke/bore ratio decreases, which makes it hard to obtain the high compression ratios required by high specific power engines.

The Beare Dual Piston Six Stroke Engine does not have these limitations, because the main lower piston does not have valve cutouts and the combustion chamber is a compact design with squish contribution from both upper and lower pistons. The shape of the Beare Head Chamber is much more like a fist than a flat hand hence thermal efficiency is high.

The Beare Head Combustion chamber diameter is approx 75mm. The Beare main piston is lighter and stronger than the 4-stroke, because the lack of cutouts allow a thinner slightly domed top. Malcolm Beare believes that the Beare Six Stroke 15kg weight advantage will be a major benefit of the Beare Dual Piston Engine, much more benefit than the 30kg handicap enjoyed by Twins in 500cc two stroke racing. "Working on the assumption that all these four-strokes are going to make enough horsepower, 15 kilos is a lot," Malcolm says. Its straightforward enough, the Twins will have a 10 percent weight advantage and force equals mass times acceleration, so it is a big difference.

Sixstroke Beare 900cc Vtwin MOTO GP

Bore: 116.25 mm
Stroke: 42.5
Upper bore: 82mm
Upper stroke: 34mm
Compression ratio 12.25 to 1
Power: 337HP @ 15000 RPM
Torque: 74.6Ft/Lbs x80% x2 = 118Ft /Lbs
Piston speed at 18000 is 5019 Ft/min or 25.4965 Metre/sec

XL engine file
Torque 101.2 NM or 74.6 Ft /Lbs discount by 20% and multiply by 2 for twin cylinder is 118 FT/ Lbs
6 port design with 3 exhaust ports leading to a rotary disk, 3 intake ports, One intake rotary disk and 2 reed valves with air assisted injectors. 2 or 4 10mm plugs per cylinder.
The port area is oprox 20% to 30% more than a 4 valve head

Results of XL file sixstroke touque calculator

Based on Dual Cycle
Total Torque
Four stroke: 62.00

Six stroke:
Main: 66.05
Top: 35.15
Total: 101.20

Increase in torque: 63.23%


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