Power Bicycle

The Ultimate guide to Motorized Bicycles

cnc cylinder head

CNC cylinder heads

Introduction

Increasing the compression ratio of a two-stroke motor certainly improves the theoretical efficiency of engines. In practice, however, it is not a matter of simply “skimming” the head as you would with a four-stroke engine. In tests, skimming the head of a two-stroke motor results in practically no performance improvement but does increase strain on the con rod and bearings shortening the life of the motor. Raised compression also increases heat in the combustion process which can lead to poorer performance and can be a fast way to melt the piston or seize the motor completely.

Overheating piston

The greatest problem is overheating the piston crown. When heat is transferred through the piston crankcase air/fuel is heated before combustion and the “charge” from transfer ports is reduced leading to a loss of power. At pressures beyond this, the piston becomes so hot below the spark point, that it can lead to pre-ignition which raises temperature further and can quickly melt the piston and kill the engine. Compressed head design in two-stroke motors is not, therefore, a simple matter. As two strokes fire at the top of every stroke, the compression ratio and the combustion area design are crucial if overheating is to be avoided. There are, however, proven cylinder head designs that effectively raise the compression (and efficiency) of a standard motor and form a fundamental building block in improving performance.

The combusion process

Thousands of an inch before the piston reaches the top of the compression stroke the compressed air/fuel is ignited by the spark plug.  We have to realize that at 4000 RPM, the piston travels from TDC to the exhaust port in 5 thousandths of a second (0.005 of a second).

 

If we look at the explosion in these time frames then the flame front of the explosion is important: After the spark, the flame spreads away from the spark plug electrode. This initial ignition is relatively slow compared to the speed of the piston. In fact, at the speed of the initial burn, the next compression stroke would be starting before the flame burns all the mixture. This is why ignition is set to spark a few degrees before TDC.

In the microseconds following the spark.  the flame front picks up momentum, spreading more rapidly. The correct ignition timing ensures that most of the fuel burn takes place when the piston is at the top of its stroke and the fuel/air compression is maximized. By the time the piston is traveling down the barrel, the fuel/air mix should be entirely burned.

Cylinder head design

Cylinder head with offset spark plug

Flame front in cylinder head with offset spark plug

From this information, we can begin to understand good and bad designs for cylinder heads. For example, offset spark plugs are poor design as they provide a relatively long and uneven burn.

Flame front in offset cylinder head

Diagram showing progression of the flame front

At high engine RPM, the mixture is not fully burned until the piston is significantly further down the power stroke.

Fuel burn has further to travel in offset spark plug positions.  This means that fuel takes slightly longer to burn and ignition timing has to be set a fraction earlier than for centrally located spark plugs. In essence, the burn is initiated in the upstroke on one side of the piston when the gasses are compressing and on the downstroke at the other side when gasses are expanding. This makes engines with offset spark plug cylinder heads more prone to “piston slap”: A rattling noise caused by the piston wall knocking against the barrel – Although disconcerting, this noise is fairly harmless and only a fractional performance loss will be associated. A more serious issue is localized heating of the piston and barrel which increases the likelihood of power loss, pre-ignition, and engine failure. Raising compression with this style head can exacerbate these issues.

Performance of offset cylinder heads

The loss of performance of offset spark plugs is shown in this diagram. Note that the higher the engine revs, the more combustive gas will be unburned 5 degrees after TDC (Top Dead Centre). In other words, the bulk of combustible gas is ignited far too late into the downward power stroke.  Engine efficiency is progressively reduced as RPM increases.
Cylinder head with centred spark plug and bell shaped squish band

Flame front in cylinder head with offset spark plug

The bell shaped combustion chamber projects the force downward. In addition, the shape of the cylinder head applies force earlier in the power cycle.

High performance cylinder heads

Centered spark plug on a cylinder head with a bell shaped squish band is the best proven design for high performance two-stroke motors. A “squish band” at the edges of the barrel not only raises compression within the cylinder head, it also forces the combustible gas into a smaller area closer to the spark point.  This provides greater downforce early in the power stroke. The smaller combustion area allows the flame front to ignite the chamber gasses more rapidly which reduces fuel/air burn duration. This design increases compression and speeds up the ignition process to produce greater downward thrust for longer duration. In short, a strong fast explosion provides a far greater downward force on the piston for more time. A WIN-WIN scenario.
High compression cylinder head

CNC cylinder head

CNC cylinder heads look macho and provide fantastic cooling for bicycle engines.

Cylinder head comparison tests

Theory predicts that central spark plug cylinder heads with bell shaped squish bands should produce more power.  In light of this, our real world tests were disappointing. We tested central mounted low compression heads against central mounted high compression heads and found that there was little to choose. We also tested the offset spark plug, high compression cylinder heads that are standard on most bicycle motors these day.  We did find that these heads are prone to pre-ignition when tuned but, other than that, there was nothing to choose between low compression heads and the offset, high compression cylinder head. We believe that the pre-ignition is caused by localized heating of the cylinder head.  The spark plug thread is machined at an angle into the head. This leaves a sharp metal edge in the combustion chamber where the last thread protrudes.  We found that chamfering the thread in the combustion chamber usually cured this issue.

Performance of CNC machined cylinder heads

Power calculations and cylinder head theory gave us great expectations of performance improvements with CNC machined cylinder heads.  The squish band is perfect and the spark plug position is correct. Unfortunately, we found no significant power improvements with the CNC heads that we were not able to gain with standard heads. CNC heads do, however, provide superior cooling and make give the engine a fantastic macho appeal.
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We compare compressed cylinder heads for bicycle engines. We test the performance of CNC cylinder heads with central spark plug compressed heads and slant spark plug high compression cylinder heads. We look at all options and test them against a standard OEM cylinder head.
Steve
Power Bicycle
Power Bicycle
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