[jessy]

This is a copy of an article i have just found while doing a search on timing problems with my saito 80 engine.
I hope it will help others if they ever take their 4 strokes apart like i did mine...ENJOY

Jesse


In the March 2004 issue, my "4-stroke Power" article raised additional questions about these powerplants and how they operate. In this issue, I'll discuss the basics of 4-stroke valve timing and answer some readers' questions.

The 4-stroke's intake and exhaust valves don't actually open and close at top dead center (TDC) and bottom dead center (BDC) (that is, every 180 degrees), even though, in theory, they do. In modern high-performance designs, engineers extend the intake and exhaust periods by opening valves early and closing them late. For example: the intake valve always opens before TDC at the end of the exhaust operation, and the exhaust valve always closes after TDC at the beginning of the intake operation. This extension results in both valves being open at the same time around TDC. Known as "valve overlap," this seems counterproductive to efficient, powerful operation, but it isn't.

There are two reasons for this extension of the intake and exhaust periods. First, poppet valves do not open and close instantly; like all mechanisms, they require time to move from one position to another. In many 4-strokes, the crankshaft must rotate as much as 30 degrees before the valve is opened to 10 percent of its total lift. Restricted valve openings restrict gas flow.

If the valves could open and close instantly, TDC and BDC still wouldn't be the best choices for their opening and closing; therefore, the second reason to extend the intake-open and exhaust-open periods involves the inertia of the gases involved. Inertia is often defined as, "A body at rest or in motion will continue in that state unless acted upon by an outside force." As the piston accelerates away from TDC at the beginning of the intake stroke, fresh air/fuel gases in the induction tract (the carburetor and intake manifold) are pushed into the cylinder by atmospheric pressure, but because of inertia, this isn't immediate. Moving slowly at first, the induction gases try to catch up with the rapidly accelerating piston; next, the piston decelerates rapidly as it nears BDC, but the mixture charge is now moving rapidly because of the inertia of its motion. If intake-valve closure is delayed until after BDC, the cylinder will continue to fill even as the piston begins its sweep toward TDC on the compression stroke. Extending the induction period maximizes cylinder packing and enhances cylinder pressure, crankshaft torque and engine power.


The principles of inertia also apply to exhaust gases. The exhaust valve opens before the piston reaches BDC (toward the end of the power event), and this allows the still-pressurized cylinder gases to leave the engine. Dumping a bit of tail-end power-stroke pressure may seem counter-productive, but the tradeoff saves some of the rotational momentum that would otherwise be used to help scavenge exhaust gases. Because exhaust-gas acceleration and cylinder scavenging begin early, the slowing of the piston near TDC allows the formation of a negative pressure zone in the combustion chamber. The intake valve is opened before TDC, and the partial vacuum promotes the delivery of a fresh mixture before the piston begins its intake stroke. Like the intake valve (which closes after BDC), the exhaust valve's closing after TDC enhances cylinder scavenging for the same reason: the inertia of the exhaust gases. The valves' opening and closing points affect the engine's ability to fill the cylinder with the fresh air/fuel mixture that is necessary for good torque and power production. The ideal valve timing will, however, depend on the type of engine under consideration by the designer: high-rpm, maximum-power-output engines require valves that open early and close late. Fuel-efficient, lower-power engines need valves that open and close closer to the dead centers with reduced valve overlap and less chance of the gases mixing and for reversion (exhaust escaping through the intake valve and into the intake tract).

With 2-stroke cycle port timing, the same events take place in the 2-stroke engine but are completed in one crank-shaft revolution; you can imagine why events overlap, gases occupy the same space and the exhaust mixes with fresh air/fuel mixture.

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[Gary Maker]

Thanks for sharing that info with us Jessy! I am a great believer in not only learning that something works, but I love learning HOW it works and WHY it does this or that!

I have taken every engine that I have owned apart at one time or another! Done some repairs or just simple maintenance and reassembled them. I know they work and do what they are suppose to! But this is the first time I have read an explaination of valve timeing and really enjoyed learning the Why and How of the valve operation! Very interesting!

Again, thanks for sharing!

The 4-stroke's intake and exhaust valves don't actually open and close at top dead center (TDC) and bottom dead center (BDC) (that is, every 180 degrees), even though, in theory, they do.

I think the above quote needs a bit of clarifying. At TDC when the engine fires surely both valves are fully closed. I have just rebuilt a number of four strokes and that is the case.

Ed S

[skypupnut]

on the compression stroke both valves are closed at TDC
on exhaust stroke both valves are opened a little at TDC

Jean

[jessy]

I agree with Jean. ON exaust stroke BOTH VALVES are opened slightly. it is called "scavenging"

[AJCoholic]

Due to the simple fact that valves do not open and close "instantly" - ie, they take a certain amount of time to get open, and then get closed, there is always some overlap. Much like in a two stroke where intake and exhaust overlaps - its much the same in a four stroke.

The easiest way to see this is to take off the rocker covers and slowly turn over the engine by hand ( in the correct direction) and visually see how the valves go up and down corresponding to where the piston is in its stroke.

Also, if you really want to get more in tune with what's happening, try and find a used copy of Peter Chinn's "Model Four Stroke Engines" - unfortunately no longer in print. Its a great read. And also, Dave Gierke's similar book on two stroke engines (still printed). WHich also describes the two stroke engine in its operation.