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Hydraulic valve control with hydraulic valve tappets
Hydraulic valve tappets enable automatic variable valve control in combustion engines since c. 1954. In the past, the valves had to be periodically adjusted with a rocker arm. Hydraulic valve tappets made this step superfluous.
Construction challenges in the engine
Principally, a car engine consists of a combustion chamber, pistons and valves. This applies to diesel as well as petrol engines. Standard engine construction places the valves directly above the combustion chambers to enable their direct control by the camshaft. This construction is still commonly used despite its disadvantages. The main problem is the fact that it makes the engine very high. Deep engine compartments are necessary to house the high engine block. This is no problem in vans and limousines. Car fronts becoming increasingly flatter, particularly in sports cars, require different engine concepts. One of the solutions was relocating the camshaft.
Relocated camshaft posing new challenges
In compact cars and sports cars, e.g. the Fiat Cinquecento or the Pontiac Fiero, the camshaft was placed beside the engine block (cam-in-block). In these engines the camshaft's mechanical power is transferred by push rods and valve rockers successively. The problem with this solution is the extension of the transmission making valve timing less accurate. To prevent the valve clearance from requiring constant adjustment, hydraulic valve tappets were invented, automatically aligning the valve clearance. Although tappets were originally intended for engines with a long valve drive, they are nowadays standard in all engines with overhead camshafts as well.
Construction of a hydraulic valve tappet
A hydraulic valve tappet is installed between camshaft and valve. The component is made up of several parts:
- pushrod cover
- check valve
- return spring
The stud, situated in the pushrod cover has a certain clearance. The space between stud and cover allows oil to penetrate. The stud is kept under pressure with a spring. A small hole in the stud allows oil from the engine to flow in. The hole is shut by the check valve through a spring pushing against the check valve. The opening can only give way to the oil when the passing cam produces a pull effect. The oil flows through the originally rigid bolt, compensating the smallest valve clearance.
Wear of a hydraulic valve tappet
The hydraulic valve tappet is a complex part with several smaller components. It depends greatly on oil always being adequately liquid and pure. Deposits accumulating in the hydraulic valve tappet will cause it to cease function. Additionally, return springs lose their tension over time. This specifically applies to the small spring keeping the stud's check valve in its place. It is important for the hydraulic valve tappet to maintain the full hydraulic tension when a cam is passing. This is impossible with a defective return spring. A defective hydraulic valve tappet makes itself heard, initially by clinking and rattling in the engine. In older cars like e.g. the Golf 2 this can be ignored. Modern engines have considerably more power and react more sensitively to all kinds of malfunctions. In worst case, the valve clearance can be compromised, causing the pistons to hit the valves which could seriously damage the engine.
Replacement of the hydraulic valve tappets
As valve tappets sit between camshaft and valves, the overhead camshaft in normal engines has to be disassembled. This includes removing the camshaft drive. In engines with a timing belt this is relatively easy. Engines with a timing chain pose a considerably bigger challenge. In both cases the occasion should be used to replace both parts as well. For engines with a belt drive this is evident. For timing chain engines it depends on the timing chain's age. Nevertheless a large part of the works is already done by disassembling the camshaft, and therefore it is really an excellent occasion. When the camshaft has been removed, the tappets can generally be pulled from their brackets with pliers. They are simply replaced by new valve tappets. Replacement of hydraulic valve tappets requires absolute hygiene! After installation of the new valve tappets and re-assembly of the entire engine, the car must be left standing for at least an hour, allowing the new valve tappets to settle. Only after this period you can start the engine and let it run idle for several minutes.
Repair of valve tappets?
Some DIY'ers attempt repairing the hydraulic valve tappets by pulling return spring and valve apart. Save from the fact that this is a lot of work, very little is gained by this. Although it might be possible for the push spring to actually seal the valve tappet from the inside again, the pressure force of the spring is no longer defined. The return valve should open at a certain oil pressure. This is no longer certain, as the spring pressure might be too high, potentially compromising the function of the hydraulic valve tappet. Furthermore, a forcibly compressed spring wears very quickly and therefore the problem is only delayed for a thousand miles at most.
Costs for a hydraulic valve tappet
The futility of a hydraulic valve tappet repair attempts becomes evident when considering the prices of the new components. A new hydraulic valve tappet from a branded manufacturer like FEBI BILSTEIN costs only 10 EUR (c. £9) even for an expensive car like Mercedes-Benz. For a large engine with six or eight cylinders the replacement of all spare parts would set you back less than 100 EUR (c. £90), causing your engine to run smoothly, ready for tens of thousands of miles of optimal service.
Sensible additional measures
The disassembly of a camshaft is an extensive operation on the engine although the cylinder head is still in place. Nevertheless it warrants a profound inspection of the camshaft. A worn hydraulic valve tappet might have lack of oil as a cause. This would leave its traces on the camshaft. This causes a considerable additional expense, though would on the other hand constitute half an engine revision. In this case, the timing chain and tensioner or timing belt with pulley and water pump are replaced as well. If the condition of the camshaft is within tolerance, at least the shaft bearing must be replaced. Used and disassembled bearings should not be used again. It is a sensible measure to supply the new valve tappets with fresh oil. A profound oil change, filter included, is recommended for every repair of this magnitude. TIP: an oil analysis prior to repair gives detailed information on the internal condition of the engine, identifying potential extensive wear of the piston gaskets or crankshaft bearings. Traces of penetrated water or an exceptional amount of soot in the combustion chamber would show. Nowadays, oil analysis enables very high precision, possibly even identifying a defective cylinder head gasket. With a small investment in oil analysis, an extensive overview of all necessary works for an entire engine revision is obtained. Otherwise, it would be a real nuisance, having to disassemble the engine again after replacing the hydraulic valve tappets.