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What is Valve Timing?

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  • Written By: G.W. Poulos
  • Edited By: Daniel Lindley
  • Last Modified Date: 03 November 2016
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In internal combustion engines, there are at least two valves for every cylinder: an intake valve and an exhaust valve. In order for the engine to operate properly, the intake valve must draw in fuel and air at the proper time, the cylinder fires, and then the exhaust valve must open to allow the burnt fuel to escape so that the cycle can repeat. For the engine to function properly, the precise actuation of these valves, in the proper sequence, is the valve timing.

In most engines, when the cylinders fire, they exert force on the camshaft, which causes it to turn and perform a number of functions. It drives the transmission and other components of a vehicle, such as the alternator and water pump. Also, as the name suggests, the camshaft has a series of cams along its length.

These cams press up on devices called push rods that in turn push up on devices called rocker arms. Rocker arms serve as levels to press down and lift off of spring-loaded engine valves. This action opens and closes the valves at the proper time so that the engine runs smoothly.

The mechanical nature of engine valve systems means that there are limited ways to adjust the valve timing. Usually, there is only one way: to disassemble the engine and reposition the cams on the camshaft that begin the sequence to actuate the valves. Other problems, however, can have the appearance of inaccurate valve timing.

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A bent push rod or valve shaft can cause a valve to not fully open or close when needed. These types of problems are not actually valve timing related, but only cause symptoms that appear to be timing related. The valves are being properly actuated at the proper time; they simply cannot perform their functions due to physical damage. These problems also cannot be adjusted for, but instead require the replacement of any defective components.

Engine ignition timing is often mistakenly thought to be responsible for bad valve timing; however, this is not the case. While it is true that defective ignition timing can cause the cylinders to fire while the valves are not in their proper positions, the valve timing itself, which is set by cams on the camshaft, is not to blame. The ignition timing is at fault.

The ignition timing is responsible for sending power to the spark plugs, which fire the engine’s cylinders. Ignition systems know when to send the power to the plugs, based on settings that inform them of the physical location of the cylinders and valves. When properly set, the ignition timing fires the cylinders at the exact moment the valves are in their proper positions.

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anon165984
Post 4

Is it required to inject the fuel after completion of TDC? If it's so, why?

anon162467
Post 3

EMD engine maintenance manual says for 710 g3b engines the exhaust must open after 107.5 deg ATDC and before 112 degrees before ATDC. On practical if it is at lower degree of open engine demanding more fuel.

poulos
Post 2

If it's a question of evacuating the exhaust gases prior to fuel injection, either expanding the exhaust valve opening to shorten the exhaust time needed, or retarding the fuel timing slightly so it doesn't attempt to inject prior to valve fulfillment, should improve the situation. It is also possible that you can have solid exhaust valve/port action but are lacking a clean and efficient intake system, or even a low ignition voltage. In the former event, expanding your intake opening should improve fuel efficiency.

Also, any air intake placement that would help generate intake air pressure using the vehicle's motion would help as well -- assuming that you are not already pressurizing intake gases already.

anon160697
Post 1

I am a engineer and we are holding around 180 emd 710 locomotive and we want to make all the locomotive fuel efficient. we are maintaining optimum valve opening pressure of unit injectors and also we are not happy with the fuel efficiency, but i believe that by adjusting the exhaust valve opening degree or advance or retarding in fuel injection timing, we may achieve it.

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