The Exhaust 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/)

This, the last of the four strokes, seems like the simplest: The job is to push out exhaust gas by moving the piston from BDC to TDC, thereby clearing the cylinder in readiness for the cycle to repeat.

First Complication

When the exhaust valve(s) starts to open some time Before Bottom Dead Center (BBDC), the residual pressure in the end-of-cycle combustion gas causes it to expand at high velocity into the exhaust pipe. The easier it is for the exhaust to leave the cylinder, the less pressure will remain to make the rising piston do work to push the rest out. Therefore the exhaust port must be designed to be as free-flowing as possible. Otherwise, power will be taken from the crankshaft to pump the exhaust out against the resistance of a poorly designed port. Roughly half of the heat picked up by the cylinder head comes from the port, where conditions for heat transfer are ideal: very high temperature and extreme velocity. This makes it important to reduce exhaust port surface area to the practical minimum—make it small, short, and straight.

Related: The Intake Stroke

Removal of exhaust gases is not as straightforward as pushing it out of the exhaust port with the piston. (Illustration by Robert Martin and Ralph Hermens/)

Second Complication

When the piston reaches TDC at the end of its exhaust stroke, the combustion chamber volume above it is still filled with exhaust product. If that exhaust gas is not somehow made to leave the cylinder, it will dilute the incoming fresh air-fuel mixture, thereby reducing the volume of mixture the engine can inhale.

How big is that volume? In a modern engine with the usual 13-to-1 compression ratio, it will be small—just over 8 percent of the cylinder volume. But the lower the compression ratio, the larger the combustion chamber volume above the piston at TDC. In the 1913 Model-T Ford, its 4.5 compression gave it a charge dilution percentage of nearly 29 percent! And if, on fine weekends, you ride a classic 1960s Triumph or BSA 650, its 8-to-1 compression will give it a 14 percent dilution percentage.

Related: The Compression Stroke

Valve Overlap

There is a way around this, and it’s called valve overlap. It was discovered before World War I at England’s great Brooklands Speedway by motorcycle tuners who were trying everything they could think of. They found that if they timed intake and exhaust valves so that the exhaust was not quite closed at TDC at the end of the exhaust stroke, yet the intake valve had already begun to open a bit, their engines ran a useful bit faster on the concrete banking. This condition, called valve overlap (both valves being open together for a period around TDC at the end of the exhaust stroke), allowed low pressure in the exhaust pipe to let atmospheric pressure push mixture into the cylinder and out through the just-closing exhaust valve. That replaced the residual exhaust gas with fresh mixture. Valve overlap is the number of degrees between the beginning of intake valve lift BTDC and exhaust closure ATDC.

Valve overlap matured into a valuable performance booster. On two-valve racing singles of the late 1930s, overlap periods as long as 100 degrees were in use, together with exhaust pipes of such length as to reflect a negative exhaust wave to arrive back at the closing exhaust valve, to do its good work:

1. Clearing the combustion chamber of exhaust gas, and
2. <i>Starting the flow of fresh mixture</i> even before the piston has started down on its intake stroke.

Because valve overlap can allow some fresh mixture to escape into the exhaust pipe (where it becomes Unburned Hydrocarbon [UHC] emissions), overlap timings are now under regulatory pressure to be as short as possible. On Ducati’s Diavel, overlap is only 11 degrees, while on the superbike V4 Panigale it is 26 degrees. In recent designs, this loss of overlap timing is compensated by somewhat increasing valve lift.

With the closure of the exhaust valve(s) just after TDC, the exhaust stroke ends. Yet the next cycle—the intake stroke—has already begun with the beginning of intake valve lift just before TDC.

Related: The Power Stroke

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