Traditionally, the valves in older engines used to get lubricated by the products of combustion. With the evolution of better fuel technology, low-leaded and LPG fuels, these produce a dryer, lubrication-free exhaust gases – resulting in unlubricated valve faces. Combined with slack tappets, weak springs or valve-to-port seat misalignment, the exhaust valve seat can suffer a heavy pounding and serious wear.
Diesel vs. Petrol Engines
On turbocharged diesel engines, the inlet valve can suffer too. The increased air pressure in the inlet port prevents the lubricating oil from accessing the air stream from the valve guide. This results in dryness and wear of the valve face and seat. However, the wet fuel in turbocharged petrol engines acts as enough of a lubricant to prevent the occurrence of any wear.
Exhaust Valve Burning & Guttering
Exhaust valves tend to go through valve face burning and guttering; the latter forming a deep channel in the valve head that develops cracks or breakage of the valve itself.
Causes of Valve Damage
The likely causes of burning and guttering include:
- Torching during erosion: of the nickel-based alloy of the valve faces, creating a distinctive deep gutter.
- Lack of tappet clearance: allowing carbon build-up on the seating face and destroying the thermal heat path outlet. This raises valve head temperature until hot corrosion or burning occurs, thereby increasing the localised gas leak and subsequent torching that creates the gutter.
- Carbon deposit build-up between valve face and seat: caused by fuel rich combustion and excessive oil consumption. It destroys the thermal path, leading to valve burn out. This too can cause a localised gas leakage and accelerated failure.
- Pre-ignition: wrong air/fuel ratio, inaccurate timing or hot spots on the periphery of the valve head produce high gas loads and localised high temperatures, leading to guttering or piston failure.
In the case of an inlet valve, the alloy carbon steel composition has the same burning and erosion characteristics as the nickel-based alloys and leads to guttering as a result of broad-face burning.
A Broader Problem
Broad-face burning is when the valve face gets burnt without any visible deep gutter or local channelling. The valve material is burnt or corroded across the exhaust face, so instead of a localised escape of gas, there’s an escape over a wider area – affecting the exhaust performance noticeably.
The problem is most likely to affect non-hard faced valves and is caused by:
- valve head/seat misalignment
- weak valve springs
- movement of the valve seat insert
Rarely can inlet valves be affected by the same reasons.
Vehicle manufacturers have made various design alterations to prevent exhaust valve face wear, such as:
- amending the valve geometry
- specifying more wear-resistant materials for valves and seats
- changing the metallurgy of the cast iron head
For inlet valves, replacement valves from reputable manufacturers incorporate proven materials, design innovations and manufacturing techniques for effective wear prevention.
Manufacturers have been continually taking steps to eliminate these issues with valves. But careful fitting and adjustment ensuring that replacement parts are only sourced from reputable manufacturers is always the best way to avoid problems. Federal- Mogul, with years of expertise and quality control modules, ensures that its valves are OE-specific and deliver to the exacting standards expected of them.