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The Power Stroke

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

The spark of the ignition begins the power stroke a few degrees ahead of top dead center. This begins the process of releasing chemical energy from the air-fuel mix. The heat of the initial spark breaks up molecule structures of higher energy, allowing them to recombine with arrangements of lower energy. Heat is the result of this difference, which releases it in the form a greatly increased molecular velocity. The temperature of this compressed mixture soars to a new level, while its pressure is roughly multiplied seven times. This is the engine’s driving force.

It is not surprising that less than 100% of the fuel burns. Some is forced into piston ring crevice volume (1–2 percent) and some is so cooled by being close to cooler metal walls (called “quenching”) that it burns incompletely. The combustion efficiency is thus in the 90s. Engineers strive to increase this number, as it reduces Unburned hydrocarbon (UHC), emissions.

In general, ignition takes place BTDC and combustion continues ATDC roughly the same number of degrees. If the best torque is at 36 BTDC then the combustion period will be roughly 36 x 2. This equals 72 degrees. In an engine having very slow combustion, such as Harley-Davidson’s first iron XR of 1970 (ignition at 50 BTDC) the whole combustion event could require 50 x 2 = 100 crankshaft degrees.

Related: The Intake Stroke

Combustion continues ATDC for about as long as the spark fires BTDC.

Combustion continues ATDC about as long the spark fires BTDC. (Illustrations Robert Martin and Ralph Hermens/).

On a graph of power stroke cylinder pressure, we can see that the ignition process is not very active for several degrees. This is due to the small size of the initial flame kernel. After TDC, pressure increases rapidly and reaches a maximum value. As the piston begins to descend on its power (expansion), cylinder pressurization falls as the volume within which the hot combustion gases are confined rapidly increases. While cylinders pressure falls, the lever arm of the crankshaft on which it acts is increasing. The torque is zero at TDC because the effective crank arm has zero length. However, as the crankpin rotates, the lever that acts on combustion gas pressure becomes longer. At about 30 degrees after the TDC, maximum torque, or moment, is achieved. Very quickly, cylinder pressure drops to such a low value that it’s more valuable to begin the exhaust process early than it is to continue the expansion of such low-pressure combustion gas. Around 80 percent the pressure energy has been transferred from the combustion to the piston at 80 ATDC.

It is important to note that the correct ignition timing can vary depending on engine design, as well as throttle opening and rpm. Functionally, it is that timing which reaches peak combustion pressure at about 11 degrees ATDC, which is roughly where the piston’s downward motion becomes significant.

The combustion time is shorter, not longer. This is because there’s heat loss to the piston crown and combustion chamber surface from the combustion gas. This is how Keith Duckworth’s classic Formula 1 engine—the Cosworth DFV—was able to show its heels to higher-revving V-12 engines with tremendously greater total valve area: DFV’s ignition timing for best torque was just 27 degrees. It was not the usual 50 or 60 degree BTDC timings of engines with deep hemi-combustion chambers and high piston domes.

Why were those designs so early? The space between a tall piston dome and a deep combustion chamber is like the peel of half an orange—so You can also find out more about As the piston approaches TDC, it quickly stops charging. It is a problem. There is no space in which a charge can be sustained There may also be deep cutaways for valve clearance in the piston dome that act as turbulence stops.

In addition to the long combustion time, surface area was another issue. The area of the deep hemi-chamber with a valve at 90 degrees included the angle. The cylinder bore is twice as largeA tall dome increases the area of the piston’s crown. In such engines, heat loss is not only prolonged (due to slow combustion), but also occurs through an excessive amount of surface area.

I always think of Germany’s wartime MG42 machine gun, whose barrel, in heavy use, had to be exchanged for a fresh, cool one every 250 shots. Why? If it gets any hotter it can spontaneously fire, with the barrel as the ignition source.

In a piston engine, the analogy is that the extra heat entering those over-large piston and chamber surfaces abnormally increases their temperature—especially if the engine is air-cooled. The hot surfaces rapidly heat fresh charge in contact with them, accelerating the pre-flame chemical reactions that can end in detonation (I almost said “in tears”). To compensate, compression ratio in such “hot” engines must be lower, resulting in a loss of performance and fuel efficiency.

Related: Compression Stroke

Preignition typically occurs near bottom dead center. This Kawasaki H2R 750cc triple piston died of detonation, destroying the top land and then moved onto ring lands. Eventually it seized.

Preignition occurs most often near the bottom dead center. The Kawasaki triple piston H2R 750cc died from detonation. It destroyed the top land, then moved to the ring lands. Eventually, it seized. (Jeff Allen /)

Remember that the words “detonation” and “preignition” describe different phenomena. Preignition occurs when something extremely hot, such as a glowing carbon deposit or an overheated electrode, ignites the mixture. Before the ignition spark. This usually occurs at the bottom center of the piston, forcing it to move. Compress a burning mixture. The piston will fail almost instantly as it heats up and deteriorates so rapidly.

Detonation is the result of a normal burning process. Small volumes of “end-gas” (unburned mixture out at the cylinder wall) having been heated to some 950 degrees Fahrenheit by all the heat it has picked up, autoignites, then burns at or above the local speed of sound, generating shocks that damage parts. We hear this as “knock” or “tinkle.”

Slow combustion also results in loss of what I will call “effective compression ratio.” Charge burning at TDC has the benefit of the full compression ratio, but charge burning long after TDC will be expanded less, resulting in its delivering less-than-maximum energy to the piston crown.

Why must everything be so complex?



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