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Posted: Thu Sep 25, 2008 9:50 pm
You choose the metal of the head based on the likely cylinder pressure, the minimum allowable casting thickness, how thick the walls between galleries etc. will be, thermal expansion properties and cost.
On some engines there are no seats as such - just a machined head.
On some the insets are soft.
On some hard.
You would often choose hardened seats if you were going to have an long-duration aggressive cam profile that would lead to a hard landing of the valve (high performance variant), and maybe go with the soft option on a soft cammed luxury model.
The choice of valve seat material (or if there is even a separate seat) is based on wear resistance, hardness and thermal stability of the seat. If you have the head expanding too much compared to the valve seat the seat will fall out of place.
However with value-engineering principles the question is not 'Whats is best?' but 'What is adequate?' or worse 'What will do the job cheapest?'
Hence a Honda will run happily for a few hundred thousand miles on unleaded petrol on its 'soft' seats, but LPG running will wear them down.
I don't think you can attribute it so much to geography, but rather to accountancy.
Posted: Sat Oct 18, 2008 3:21 pm
I have to raise most important point in discussion about wearing valve seats:
It is rather temperature factor, than chemical, which has an influence to wearing valves and seats!
Example? Ok, but let's start from beginning:
Temperature of burning mixture in combustion chamber depends on a few main parameters:
• physical and chemical characteristic of fuel
• cylinder pressure (which means in fact thermodynamic efficiency)
• speed of burning
• ignition point
1. Different fuels mixed with air (more precisely: with oxygen, the air contains) in stoichometric ratio, give different burning temperatures (it's not the same, as amount of produced energy!). Mixture basing on petrol burns with lower temperature, than LPG, what produces more heat, and of course higher temperature of exhausting gases
2. From thermodynamic point of view (see theoretical Carnot's engine model), bigger difference between beginning and final pressure in cylinder gives more energy transvested from heat to pressure. Lowering compression ratio result in higher temperatures of exhausting gases. That's a dangerous for our valves.
3. Burning speed varies pretty much depending on AFR (ait-to-fuel ratio). Riching mixture gives faster combustion, what results in more heat energy converted into mechanical, and lower exhaust gas temperature. Additionally, when using petrol, on very highly raised engine power (standard when turbocharged) - additional amount of fuel (riching mixture more, than engine needs to produce max power) is used, to let some part of heat to be utilised to vaporise this fuel. We call it internal engine cooling.
Leaning - as the opposite - tends to gain higher temperatures, but at the same time causes combusting process slowing down. At very lean mixtures all combustion stroke time isn't enough to finish process, so it lasts even, when exhaust stroke begins, and exhaust valve is opened. This is most dangerous and most popular cause of burned valves and seats. Please note, that I've not specified any fuel here, as a factor influent for this process. You can kill your valves as well on both: petrol and LPG, when your fueling system will not control AFR properly.
4. Ignition point is important to be setted properly, to gain maximum cylinder pressure during combustion in proper point of combustion stroke. This gives maximum torque on crankshaft. Retarding ignition point tends exhaust gases to be hotter, advancing tends to knocking - on petrol more, on LPG - less.
And now, You see everything. Most important thing is to keep AFR ratio proper - as factory setted. LPG conversions often use poor fitting stuff (especially too small or bad working reducers/vaporisers), which can't keep gas pressure stabile on higher boosts. This results leaning mixture above correction limit of lambda circuit and in result - working on lean mixture especially when we most need rich one!
Properly installed LPG system should be precisely calibrated to be as close to factory fuel settings on each situation, as possible. To make it perfectly, following conditions should be kept in mind:
- Perfectly working petrol fueling control, especially engine temp. sensor, MAF/MAP, and lambda circuit.
- Enough amount of flying LPG on all conditions, including long-term WOT (wide open throttle) - good reducer,
- Precisely calibrated/fitted injectors to get proper injection times for every condition (as well idle, as WOT),
- Using of good LPG ECU with good correction aghorithms for engine and gas teperature, and gas pressure,
- Precisely setted LPG injection map. That's why I don't recommend BRC Sequent or Prins VSI systems, where setting map up is done automatically, using mathematical way instead of live map building on all range of conditions.
Above info should let everybody for enjoying with driving on LPG without costly repairs of cylinderheads, without any flashlube! In my opinion, it's infulence on engine is marginal - more psychological, than real. Additional thing for LPG traders to earn money on...
Posted: Sun Oct 19, 2008 4:45 pm
Excellent technical posting.
I agree with some of the things you have said on temperature, except the lower compression ratio (I think you meant raising CR - higher CR = higher theoretical gas temperature before combustion = higher temperature before blowdown).
I do not agree with your conclusion about flashlube though.
I agree the temperature has a big impact on the valve and valve seat, and that all the settings you have listed will have an important part to play in making for a good running engine.
From my own research when working in previous petrol and diesel engine development jobs the gas temperature at the begning of blowdown tends to have a much larger effect on the valve itself rather than the valve seat.
But anything that affects the valve must have some effect on the seat as well.
There is still the fuel lubrication issue on valve seats and valve heads using LPG. And this will affect some engine, but not all.
There is also an upper cylinder lubrication issue with lpg, around the band in the cylinder where the piston rings sit when the cylinder pressure is highest and the side thrust in the piston is high, but it's effects are tiny and probably only and issue with high mileage engines (250,000 miles - 400,000km and higher). And that is independent of any valve questions.
This is why if there was doubt over seats hardness at temperature, or how long I was going to run the vehicle then I would still favour the flashlube.
Does your business keep records of the engine types you fit LPG to and the mileages the customers are doing?
That plus some tappet clearance data for the cars would be an excellent research source for answering this question.
Maybe a valve check at installation and one at every service?
I would be happy to help develop a database for this sort of project and analyze it. It would be very interesting.
Posted: Wed Oct 29, 2008 10:00 pm
About lower compression ratio:
It was not an error - I do wrote, what I meant. That's true, that higher CR gives higher temperature before blowdown. But it doesn't raise exhaust temperature
!!! Things are exactly opposite. It is clearly shown on Carnot's theoretical thermodynamic process in gas-pressure-powered engine.
Higher compressing of some given amount of gas (here: air) gives of course higher temperature on highest compressing point, but it's influence for exhaust temperature is minimal. As oppsite to thermodynamic efficiency of conversion heat energy into pressure, after blowdown
. This efficiency raises proportional to pre-pressure (before blowdown) - and that's why higher compressed engines give more power from the same displacement. I mean, that more combustion energy is transferred into engine's power, less of it is lost through exhaust valve. Believe me - summary of energy produced on flywheel, and caughed through silencers MUST remain
the same , with no difference, how high the CR is - You know well, that energy will not be created from nowhere, it is only transferred from one form to another. So better is to get more power, and more cold exhaust gases, by raising CR.
I'm not sure, I wrote this clearly enough, but I believe, You understood me
Do widzenia ("see You later") (Dobranoc means exactly "good night"
Posted: Wed Oct 29, 2008 10:18 pm
Sorry, for adding post one by another, but I can't edit my own posts
About flashlube once more:
Runningoldcars, You wrote about lubrication issue when running on lpg - as weel for valves, as for piston's crown (first compression ring's gap and top of cylinder liner. Please, answer for three questions:
1. Would You show me difference between running on LPG and gasoline, connected with lubrication of these areas? In my opinion fuel takes no any effect of lubricating these places - there's no any friction between valve and seat, so why to lubricate them?
2. Temperature of piston's crown, combustion chamber and first ring does't allow gasoline to remain liquified, when it runs into the cylinder, if even some portion will not evaporate on intake valve's surface, during injection - so how it may lubricate something?
3. Can You show any lubricating attributes of gasoline? In my opinion influence of gasoline for lubricating processes is negative - especially, when engine is cold, and not fully evaporated, gasoline washes oil film out
from cylinder walls!!! That's why You can save a part of engine's life, switching it into LPG as soon, as possible...
Posted: Sun Nov 02, 2008 4:07 pm
Sorry for the late reply but I have to go away for business.
I will answer you properly when I come back from my trip.
I'll show you some experimental results from engine testing my researchers have done, and show you why the Carnot cycle is not really relevant. (but good for teaching with)
I will answer your lubrication questions when I have looked out the necessary SAE and IMechE papers so I can show you. I think the Russian journal Energetika may have had something as well, and Prof. Gyorgy Sitkei in Budapest looked at it as well (I may be wrong on this last one).
Posted: Tue Nov 11, 2008 12:15 pm
Dzien dobry, my Friend
As You see - I'm coming here not often too - so I can wait for Your reply as long as needed.
Best regards for You.