Good cylinder filling, even in excess of 100%, is no guarantee that an engine will make good power.
The challenge is to convert linear piston motion into rotary action of the crankshaft. The shape of the combustion chamber is critical. Various shapes have been used; flat head (side valve), hemispherical and wedge for overhead two valve engines and pent roof for multi-valves. The common design objective is to burn all the air/fuel mixture while avoiding excess heat.
Before we look at problems that can reduce our motorcycle engine's power producing capability, we need to understand some basic principles.
While certain side valvers can achieve respectable performance, the combustion chamber is not a good shape. Overhead valve engines could be produced at reasonable cost while the head and combustion chamber, being a better shape, also improve cylinder cooling efficiency. The combustion engine is a heat engine and it's highly important to retain all possible heat generated during combustion, without overheating. Our engine designers normally chose a spherical combustion chamber which, given a specific volume, offers the smallest surface for thermally conducted heat loss.
In practice a perfect sphere was rarely possible on early motorcycles, so the compromise, which is the hemispherical chamber, with valves at considerable angle to each other, is considered an efficient design and common on most old motorcycle engines.
Some early motorcycle engines employed dished-top pistons in conjunction with hemi-chambers in an effort to provide an almost perfectly spherical chamber, however this idea was short lived as the need to run higher compression ratios took precedence. Thorough mixing of the air and fuel is achieved by positioning the inlet port's angle, shape and downdraught in the optimum position to create a degree of swirl above the rising piston.
The term 'swirl' is preferred to 'turbulence', which implies movement without pattern. Swirl is essentially a charge that rotates concentrically about the axis of the cylinder, commencing on the inlet cycle and continuing into the compression stroke, thus improving the mixing process and maximizing combustion. The shape of the piston crown also plays a vital role in maximizing swirl.
Another form of 'managed mixture momentum' is 'tumble', which is common in four-valve, pent-roof combustion chambers.
Squish band combustion chambers are common on high performance two-stroke and four-stroke engines. Squish refers to an area between the flat of the piston and te flat of the cylinder head at TDC. As the piston rises on its compression stroke the mixture is squished towards the remaining space of the combustion chamber where the spark plug and valve reside. The squishing of the mixture creates turbulence and promotes a better and more complete combustion process, which also reduces emissions. Typical figures for the squish gap are around 1.01600 millimeters with high performance engines running considerable less.
For maximum power, the air/fuel mixture – ideally 14.7:1 – must be compressed. Within limits, the higher the compression ratio the better, since the mixture's vapor molecules are closer together, giving more rapid and effective combustion and therefore greater combustion heat produced. This is the compromise, where the maximum heat is retained in the cylinder for power production, yet the cylinder temperature is kept low enough to avoid distortion of components followed by seizure. To achieve high compression ratios, engines use domed piston crowns, but make the dome too big and there's a danger that the spark plug will be masked.
The plug should be positioned to enable the flame front to reach all parts of the chamber at desired point. Too close proximity of the piston crown to the plug's electrode blocks the rapid spread of the flame front, which is vital to good power production, particularly at higher engine speeds.
Advancing the ignition or machining a third pocket into the piston, near the plug position, can appear to eliminate the condition, but running a slightly lower ratio would actually produce better results.
The ideal compression ratio is the highest the engine will run on its designated fuel without distress. Most cruiser small engine four-stroke engines, have a design complication in that, to achieve good cylinder filling, the ports and valves have to be large. But with limited space in the combustion chamber, the plug is seldom in the optimum location.
It's important to keep all the heat in the right place. High combustion temperatures are good, but cylinder temperature must be kept low and air-cooling the cylinder with massive fins and aluminum is relatively straightforward. It is easy to overcool the engine, which limits its power producing ability. The reason why finning reduces towards the bottom of a cylinder is not aesthetic but simply because it requires less heat dissipation.
While cylinder heat design is the major factor in producing linear motion, converting that to rotary motion brings a lot of other components into play. Crankcase and cylinder barrel stiffness is vital. Likewise crankshaft design is crucal to manage the inertia loadings created by combustion chamber pressures. These forces are extremely high so for the crankshaft to produce maximum engine preformance it must be as rigid as possible yet capable of absorbing moments of compressive, tensile and torsional stress without breaking. Think about that for a moment. Needless to say, all reciprocating components must run with the minimum of friction.
The conrod ration also plays a major part. The shorter rod has two dramatic effects on cylinder filling. Firstly, the point of maximum piston velocity moves closer to TDC and the piston moves away from TDC quicker, creating a stronger intake pulse. The point of maximum piston velocity influences the design of the camshaft, in order to optimise the inlet charge.
Secondly, the increased angularity of the shorter rod also increases the turning effect on the crankpin during the early part of the power stroke, thus improving the engine's performance.
So there you have it. Get as much mixture in as possible, convert it efficiently into rotary motion, reduce emissions and everybody's happy.