Auto-LPG Conversion Thread

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Any unleaded capable engine does NOT need any "lubrication" of the valve seats. Leaded fuel used to serve as the protection of exhaust valve seats (due to high exhaust temperatures, compared to much lower temperatures from the intake mix). With modern engines, it's a non-issue. A lot of cities (such as Hong Kong) run their taxi fleets on LPG.

Two-stroke petrol engines (because the fuel serves as the lubrication itself) and diesels (for the high-pressure injectors) need lubrication in the fuel (diesel is actually an oil). Four-stroke petrols take in a fuel/air mix, which does not have ANY opportunity to lubricate anything and adding oil to it will NOT have any benefit (unless being invisible to the car behind is a benefit to your particular application, for those of us who aren't James Bond, no oil in petrol = good).

LPG will require a few tuning changes for the most benefit, but the same can be said for ethanol blends or high-octane fuel. Ignition timing and correct air/fuel mix is a bit different with LPG than it is with petrol, among other things. However, LPG and petrol are fundamentally the same in how they work (or shall we say, they work in exactly the same way, considering that they fuel the same engines). Driving style and conditions (whether you drive hard, stop go traffic, crusing, etc.) is a red herring. LPG vs. petrol is really more along the lines of regular vs. unleaded or 100% petrol vs. ethanol blend. As opposed, of course, to petrol vs. diesel which use DIFFERENT engines that are PRINCIPALLY different hence require different driving styles.

What I'm saying is that among the many practical reasons NOT to get an LPG kit (such as lost trunk space and high initial cost which may take too long to recover), driving style/conditions and valve seat wear are not in that list. We don't use LPG kits ourselves - because we need the space and don't run around much.

The Early Petrol Engine
One of the first things that early engine designers noticed was that valves and valve seats wear out. Hardly earth shattering news because all highly loaded mechanisms wear out of course but exhaust valves and seats tended to do it rather fast. Early engines were made out of cast iron which is a pretty decent structural material but not ideally suited to the temperatures and loadings experienced inside a combustion chamber. Valves were, and still are, made from various grades of steel. Valve seats were machined directly into the cast iron parent material of the cylinder head. The problem that became apparent was that the material comprising the valve seat gradually wore away leading to the valve sinking deeper into the "throat" of the port. This reduced and eventually eliminated the necessary valve lash clearance in the valve opening mechanism leading eventually to the valve not closing at all and the seat then burning out very quickly indeed. Research into this wear, or "erosion" revealed what was actually happening.

The Mechanism Of Valve Seat Erosion
The rate of wear was far higher than pure frictional or mechanical considerations alone could account for. If one were to construct part of a dummy engine consisting of just a valve and a cast iron seat and cycle it open and shut millions of times on a test bench, very little wear would take place. What this fails to account for is the temperatures that are reached inside a running engine. Copyright David Baker and Puma Race Engines

Components inside the combustion chamber of a running engine are exposed almost continually to the heat of the burning air/fuel mixture. In the absence of any cooling this would be enough to melt the hardest steel. As it is, most of the heat is transferred away into the cooling system or flushed out as hot exhaust gas and the steady state temperatures of the various internal components obviously stay well under their melting points. The hottest component is the exhaust valve and its seat. All the heat picked up by the valve head has to be conducted away though the seat and to a lesser extent through the valve stem into the guide. The thinner the valve seat the less area there is for this heat to be conducted away - something many race engine builders ought to bear in mind. Temperatures in these critical seat areas can reach 800c which is enough to make steel glow red. This is still well below the melting temperature of steel (or iron) but enough to soften it somewhat and significantly reduce its strength. Part of the increased wear rate can be explained by this reduction in the material strength but it still only accounts for a fraction of the wear experienced. To understand the rest we need to make a quick trip to Japan.

In conventional welding it is necessary to melt both the parent material and the filler rod. A "weld puddle" is formed directly under the torch or arc in which the two materials melt, run together and then cool as one. There is another way of joining metals without melting them though. When a sword is made it is heated till it glows yellow and then hammered into shape. As the strip gets thinner and wider it is folded over onto itself and then heated and hammered again until the two layers join together and the process repeats until many layers have been rolled over and forged into one. The process is one of heat and pressure which together achieve a similar result to that of the higher temperatures of "melt welding".

Now back to valve seats. The temperatures over a seat won't be the same everywhere. High spots and minute flaws in the material can reach higher temperatures than their surroundings because they are less well cooled although they still won't be anywhere near melting. Under the pounding of the valve these imperfections form microscopic pressure welds between the valve and its seat just like the forging of a sword which then get burst apart next time the valve opens and a speck of material is lost. Each weld might be too small to see but over millions of cycles they combine to form erosion which wears away the seat and pits the valve. Copyright David Baker and Puma Race Engines

The higher the temperatures inside the combustion chamber the more these welds are created and the erosion speeds up. So a very important factor is how hard the engine is used. At low rpm and small throttle openings there might be very little wear even when the materials are not ideal because valve and seat temperatures stay low. Under more severe operating conditions the wear rates can increase exponentially. What might be deemed acceptable wear rates then depend very much on the expected life of the engine and its operating conditions. Inlet valves are not generally a problem in any case because they run at much lower temperatures than the exhaust ones.

Solving The Wear Problem
The first and most obvious thing to do is use tougher materials for the valve and seat to resist the wear. Exhaust valves already had to be made of a very tough steel just to withstand the operating temperatures and it was the cast iron seat in the head that was the primary concern. Machining a recess into the seat area and pressing in a tougher steel insert provided an easy solution to the problem but was expensive. So research took place into additives to the fuel to find out if anything would help resist the erosion process more cheaply without modifying the cast iron seats. This also came about as part of the research into raising the octane number of fuels to enable higher compression ratios to be used without detonation and hence increase power and improve fuel economy. A substance that stood out as being very effective in both raising the octane number of fuel and also preventing seat wear was tetraethyl lead (TEL). How exactly it reduces the erosion is not clear but it seems that a thin coating of this material on the seat prevents the microscopic welds from forming. TEL was added to petrol in amounts up to about 4 grams per gallon depending on the level of octane boost required. As concern about lead emissions grew, the amount of TEL per gallon dropped until finally unleaded fuel became mandatory.

From http://www.pumaracing.co.uk/unlead01.htm