This compression action serves to provide a higher pressure against the piston when the fuel is ignited as well as pre-heating the mixture to assist with an efficient burning of the fuel. The exhaust cycle has the piston again rising upward in the cylinder while the intake valve remains closed and the exhaust valve is now open. The pressure created by the piston helps force the exhaust gases out of the cylinder through the exhaust valve and into the exhaust manifold.
Connected to the exhaust manifold is the exhaust system, a set of pipes that includes a muffler to reduce acoustical noise, and a catalytic converter system to manage emissions from engine combustion. Once the piston reaches the top of the cylinder in the exhaust cycle, the exhaust valve begins to close and the intake valve starts to open, beginning the process over again. Note that the cylinder pressure on intake helps to keep the intake valve opened and the high pressure in the compression cycle helps to keep both valves closed.
In engines that have multiple cylinders, the same four cycles repeat in each one of the cylinders but are sequenced so that the engine proves smooth power and minimizes noise and vibration. The motion of the engine valves is driven by the camshaft of the engine, which contains a series of lobes or cams that serve to create the linear motion of the valve from the rotation of the camshaft.
The number of cam lobes on the camshaft is equal to the number of valves in the engine. When the camshaft is in the cylinder head, the engine is called an overhead cam OHC design; when the camshaft is in the engine block, the engine is called an overhead valve OHV design. Regardless of the engine design, the basic movement of the engine valves occurs by the cam riding against a lifter or a tappet that provides a force that presses against the valve stem and compresses the valve spring, thereby removing the spring tension that keeps the valve in the closed position.
This movement of the valve stem lifts the valve off the seat in the cylinder head and opens the valve. Once the camshaft rotates further and the cam lobe moves so that the eccentric portion is no longer directly in contact with the lifter or tappet, the spring pressure closes the valve as the valve stem rides on the centric portion of the cam lobe. Maintaining the proper valve clearance between the valve stem and the rocker arm or cam is extremely important for the proper operation of the valves.
Some minimal clearance is needed to allow for the expansion of metal parts as the engine temperature rises during operation. Specific clearance values vary from engine to engine, and failure to maintain proper clearance can have serious consequences to engine operation and performance.
If the valve clearance is too large, then the valves will open later than optimally and will close sooner, which can reduce engine performance and increase engine noise.
If the valve clearance is too small, valves will not close fully, which can result in a loss of compression. Hydraulic valve lifters are self-compensating and can eliminate the need for valve clearance adjustments. Modern combustion engines can use a different number of valves per cylinder depending on the design and the application. Smaller engines such as those used in lawnmowers may have only a single intake valve and one exhaust valve.
Larger vehicle engines such as 4-, 6- or 8-cylinder engines may use four valves per cylinder or sometimes five. Engine valves are one of the components in internal combustion engines that are highly stressed.
The need for reliable engine operation dictates that engine valves be capable of exhibiting resistance to repeated and continuous exposure to high temperature, high pressure from the combustion chamber, and mechanical loads and stresses from the engine dynamics.
Exhaust valves, by contrast, are exposed to higher levels of thermal stress by being in the pathway of the exhaust gases during the exhaust cycle of the engine.
In addition, the fact that the exhaust valve is open during the exhaust cycle and not in contact with the cylinder head means the smaller thermal mass of the combustion face and valve head has a greater potential for a rapid temperature change. The engine will not be as powerful if a valve is damaged, so if your car starts to lose power, this is a sign of a valve problem. You may also hear a tapping or ticking noise in the engine if your car has a bad intake valve or exhaust valve.
The noise may get louder and faster when the vehicle is accelerating. The check engine light can turn on for a number of reasons, including damaged intake and exhaust valves. You will need to get a mechanic to inspect your vehicle and diagnose the issue.
Bent or burnt valves can also cause your engine to misfire. This means the engine may jerk, stall, or hesitate when your car is in motion.
These are some of the many symptoms of a bad exhaust valve or intake valve. If you spot any of these signs, contact a professional mechanic as soon as possible. If you suspect a problem with an intake or exhaust valve, you should not drive your vehicle until the issue has been addressed by a mechanic. Bent or burnt valves could seriously damage your engine if they are not repaired right away. The first step of diagnosis when damaged valves are suspected would be a leak down test.
During this test, the engine is rotated until top dead center of the currently tested cylinder is achieved. The intake and exhaust valves will be closed at this time. Compressed air is injected into the cylinder through the spark plug hole. The ability of the cylinder to hold pressure is then measured using a leak down gauge. In some cases, the valves can to be inspected using a borescope inserted into the cylinder.
Removal of the valve cover to inspect the valve train may also be required. If all else fails, the cylinder head may need to be removed to check the valves.
Your mechanic will know what to do to diagnose and repair the issue. All you will need to do is tell them what issues you are experiencing with the car. Then, they can complete a thorough inspection, diagnose the problem, and make all necessary repairs. Unfortunately, repairing bent or burnt valves is not easy. Valve replacement is a very large job requiring engine tear down. This estimate does not include the cost of parts, which will increase the total cost of repairs even further.
Sometimes, the valves may simply need to be adjusted rather than replaced. There are two types of engine valves; intake and exhaust valves. Privacy policy and terms of service Advertise Copyright EngineBasics. Advanced Tuning. Advertise with us. Contact Us. Nissan sx. Camshaft Understanding. Intake Runner Length.
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