What is a Valve?

To properly consider the Valve Seat Recesssion (VSR) phenomenon we must first ensure that the term 'Valve' is correctly understood.

Engine valves have 2 completely seperate parts, the first being the Head and Stem Assembly and the the second being the Seat.
Many will look at a head and stem and call that a 'Valve' although that is only part of a valve.

It is very important to make this distinction. VSR, as the title suggests, occurs only the valve seat, not the head and stem assembly. Many engines are produced with hardened valve seats although the stem (and possibly the head) are often not as hard. Confusion can result when considering which parts could suffer damage, and which parts may need to be harder for correct operation, especially as some anecdotal evidence comes from people claiming they 'had trouble with their engine valves'. That's fine as far as it goes, but which part of the valve did they actually 'have trouble' with?

When the term 'Valve' is used in this item, it must be taken as meaning an entire valve, i.e. the Head / Stem assembly AND the Seat, although when referring to VSR we are only considering the effects on the Seat(s). Problems with the valve head and seam assembly can occur too, particularly in the form of Valve Head Deformation, but that subject needs an entirely different description and is not dealt with in this item. In the future I may write a seperate item on Valve Head Deformation, although the aim of this excercise is to focus on Valve Seat Recession only.

What is Valve Seat Recession?

Valve Seat Recession (VSR) is the tendency for the valve seat sealing face to shrink upwards into the cylinder head. When this occurs, the valve heads and stems are pulled further and further up into the cylinder head in order to meet the shrunken (recessed) valve seats by the powerful springs which are used to close the valves. You could describe this as a kind of valve sealing self-adjustment.

Eventually, the point is reached where the valve head and stem has moved up so far that the top of the valve stem is touching the cam follower, tappet or whatever for all of the time. There should be a clearance gap here which normally ensures full and correct sealing of the valve. If there is no clearance gap, the valve may be held slightly open by the contact with the cam follower or tappet. If this happens the cylinder involved cannot be truly gas tight and the condition will result in the affected cylinder producing far less than its normal power, or producing no power at all. This is because the affected cylinder will have low (or no) compression due to the leakage at the valve.

What are the causes of VSR?

The short version(s);

There are at least 3 reasons why engine valve heads and stems recess into their seats;

1. Brinelling (flowing or change of shape of the metal that makes the valve heads and seats)

2. Erosion (through friction, resulting from the rotary motion of the valves when closing, caused by the helical valve spring(s)

3. Microwelding (ask your local Metallurgist or research on the 'net)

Note that these factors and their combined effects are present when an engine runs on ANY fuel.

A breakdown of these effects;

Brinelling (factor 1).

Brinelling is well demonstrated by a Blacksmith who heats the metal he wants to re-shape in order to make it flow (Brinell) more easily under his Hammer. The metal being worked on is said to become 'softer' (more Ductile) when it is heated. The more the metal is heated (up to the point where it becomes molten or liquid), the softer it will become. The Blacksmith could re-shape the metal when it is cold although it would take him far longer. From this we can see that more heat can accelerate the rate of Brinnelling.

In addition, the rate of change in shape or flowing of the metal depends on its hardness and ductility at any given temperature. Lead, Copper or Gold would be easy to work when cold, whereas Tool Steel would have to be glowing bright yellow with heat before a heavy hammer could make any dent in it. Metal hardness is therefore a factor directly affecting the rate of Brinelling.

In a Piston engine, the valve heads are taking the Blacksmith's place, hammering on the valve seats. The softer those valve seats are, the more they will change in shape as their metal flows. As in the Blacksmith's Forge, it is clear that the rate of Brinelling can be accelerated by more heating of the valve seat and the force exerted upon it by the valve steam /head.

To carry the useful Blacksmith analogy a little further, the effect of Brinelling can be increased by using more force or a heavier hammer (which amounts to the same thing). Engine designers would like to make their valves as light as possible in order to reduce the force, although they have to be made of enough metal to do their job adequately. One way of reducing the force exerted on a valve seat by the valve head (the Hammer) is to use two valves to do the job of one. This will almost halve the weight of individual valve heads. This solution is demonstrated in engines having 3 or 4 valves per cylinder (called Desmodromics). Many think that 3 or 4 valves per cylinder are used to improve the 'breathing' or gas flow of an engine where, in fact, the method is used to reduce the weight of the valve heads and thus reduce the force exerted on their valve seats. If this design solution is used it will minimise the effect of Brinelling and retard the onset of VSR.

Erosion (factor 2)

This effect cannot be avoided by using any pure fuel, it is purely a mechanical problem which can only be overcome by stopping the valve heads and stems rotating. When the heads rotate, they simply rub metal off both mating faces, and the softer of the two will suffer more erosion. Efforts to resolve this problem are made by engine designers, the most common solution is to use two springs with opposing helixes. Because one spring must be smaller in diameter than the other (one spring must fit inside another in an axial fashion) then the turning forces cannot be totally balanced and valve head/stem rotation will still take place, although it will be reduced.

Another partial solution for Erosion is found in lubrication of the valve head and seat. If we apply lubrication to surfaces that are eroding each other then less molecules will be worn off each surface. Lead used to be added to petrol because it provided a form of lubrication for the valve head and seat sealing faces. Nowadays, Lead is not added to Petrol so that lubrication benefit is gone.

Microwelding (factor 3)

Metallurgy shows us that any two similar metals that are truly clean (right down to a molecular level) will begin to stick back together if left touching for any length of time. The longer they are left this way (when the engine is not running), the more similar the metals are, and the cleaner their surfaces are, the more quickly they will begin to Microweld.

When two Microwelded surfaces are pulled apart, molecules of metal are pulled from both, resulting in surface damage. After prolonged surface damage due to Microwelding the valve head and seat may not produce a gas tight seal any more. The simplest way to prevent or retard Microwelding is to add 'Dirt' to the surfaces of the two pieces of metal. That 'Dirt' could be the Oil we use to stop things siezing up, it could be Polish or Paint of some kind, it could also be the Soot produced by burning some fuels. It is convenient that Petrol combustion produces lots of carbon (Soot) whereas it is a little inconvenient that LPG burns much cleaner than Petrol and does not leave Carbon deposits. Thus, the cleaner combustion of LPG and other fuels (CNG for instance) can actually promote Microwelding.

Why do some engine types suffer VSR when others do not?

If the factors above have been understood it is clear that there are two differences between running an engine on Petrol or LPG.

Microwelding

In essence, valve wear will occur slightly more quickly when running on LPG simply because it burns much more cleanly. There is no Soot or Dirt to prevent the occurrence of Microwelding. Don't let that worry you, this effect alone is very small. We are still talking about Valve life that is often longer than the life of the car.

Microwelding can be avoided, at least in part, by having the valve head and seat made out of metals that are as dissimilar as possible. Carbon steel valves and Stellite seats provide a good solution here. Another weapon in our anti-Microwelding arsenal is provided by fuel additives. These often come in the form of light Oil which, if introduced in the correct dosage can lubricate the valve head and seat faces, preventing Erosion. Also worth mentioning is that any lubricating oil will eventually burn. When it does, it will replace the missing Soot (Carbon) that can do a lot to prevent Microwelding. Injection of these tiny amounts of oil must have some cooling effect on the valves (like petrol does) although it is probably so small as to make no difference to valve temperatures.

A word of warning here - If a modern engine is Overdosed with proprietory 'Valve Saver' compounds, its exhaust emissions may well be compromised, and damage to the catalytic converter can result.

Running Temperature of the Valve Heads and Seats

The most common misconception you'll hear about LPG is that it makes an engine run hotter than Petrol. This is no more than myth.

LPG has only 85% of the calorific value of the same amount of petrol, so it is impossible for it to create more heat. It can only make 85% as much.

There is one factor missing though - The cooling effect that squirting wet petrol mist into the cylinder has on the valves. The evaporating petrol mist will cool the valves very slightly but that cannot happen when injecting a gas that has already been vaporised before injection. When running on LPG the gas does not have quite as much cooling effect on the valves. As a result of this difference you'd expect to see LPG operation showing slightly higher valve temperatures, but overall that isn't the case. We just established that LPG can only produce around 85% of the heat that petrol can, the lost benefit of petrol mist cooling on the valves is cancelled out by the cooler combusion temperatures of LPG - in short, both fuels end up roughly even when considering valve temperatures during operation.

Even if valve temperatures were to end up slightly higher, that would only present more risk of VSR if the valve seats were too soft for the job (Go back to the Blacksmith analogy in 'Brinelling' (factor 1). Engines that suffer the most are some of the Ford and Honda engines, ALL Jaguar V6's (actually a Ford engine) ALL Subaru engine variants etc. although over 90% of engines made today are just fine.

This is in no way intended as a comprehensive list of unsuitable engines and it would be wise to check with both the manufacturer of your engine and your LPG installer before conversion to find out if your engine has suitably hard valves. The UK LPG website has a list of engines known to have soft valves on a 'Caution' list, but this is not accessible to the general public. In that case it is neccesary to contact your installer prior to booking a conversion so they can check for you. If you are going to have your car converted by us then we'd be pleased to check on that for you (See our 'Information Sharing Policy' the the foot of this page).

What kind of engine is least suitable for LPG conversion?

The worst kind of engine for LPG conversion would have badly designed (soft) valve seats which will not resist Brinelling and Friction as well as harder seats would. In addition, it is known that the more similar the metals that make the valve heads and seats are, the more Microwelding will occur. Weight of the Valve Head and Stem is also as important as the hardness of the valve seats. An engine with 3 or 4 valves per cylinder is more likely to resist VSR than an engine with only 2 per cylinder, although if those 3 or 4 valve seats are soft, there will be no benefit. If the engine does not have hard valves it is wise to begin by having the valves re -engineered to the required standard using parts made of much harder material (often 'Stellite'). In addition, an engine with single helical valve springs will suffer more Erosion due to Valve rotation than an engine with two opposing helical springs would.

Can an Engine with Hard Valve seats be damaged?

Valve damage can occur when running on any fuel.

If the valves are made to run too hot due to incorrect fuel/air mixtures this may make Brinelling and resulting VSR much more likely. Incorrect fuel/air mixture, especially if too weak, can overheat the valves, resulting in accelerated Brinelling. It is important for the correct mixure to be supplied to the engine whether it is being run on Petrol, LPG or any other fuel.

Fitting Harder (normally Stellite) Valve Seats

This needs the services of a specialist engine re-manufacturer (re-conditioner). It is a wholly seperate business, requiring a large investment in machinery and the skills required. We cannot offer this service and it is unlikely that any other LPG conversion company will be able to do so. If you decide to go ahead with conversion of an engine with soft valve seats it would be best to begin by having this work done prior to conversion. A search for local engine re-manufacturers (perhaps in Yellow Pages or on the internet) would be the way to go.

In Conclusion

It would be incorrect to heap all if the blame for VSR on LPG.

If an engine's valves are truly fit to run on modern unleaded petrol (for any reasonable length of time) they are good enough to run on LPG with no additional problems, and it must be remembered that all of the effects described above still occur when running an engine on Unleaded Petrol, especially if the valves have been 'engineered down to a price'.

The plain fact is that ALL engine valves will reach the end of their useful life at some point. Nothing lasts forever, although valves with harder seats made out of quite different material to the head and stem assembly will definately last longer when running on any fuel, as will desmodromic valve arrangements.

Finally, note that most modern LPG systems depend on the car's original [petrol] fuelling system to deliver the correct mixture at all times, e.g. if the petrol system delivers weak mixtures that is a problem in itself, and will be translated directly into weak mixtures when running the engine on LPG simply because the LPG system is doing what it is told to do by the petrol system. In a case like that, the LPG system and the installer cannot be held responsible - it is an ordinarly (as if it were unconverted) car maintenance issue, not an LPG issue.

Steven P. Sparrow BSc (Hons)

February 2008

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