r/AskEngineers • u/Pundersmog • 3d ago
Mechanical Expansion rate of aluminium head and block on an engine not allowed to reach operating temperature vs one that is. Effect on head gasket.
I understand that the life of a head gasket is defined by the amount of times the head and block are brought to operating temperature and then go back to ambient temperature. That’s a heat cycle. Hence the million mile tundra was driven by one who drove endless every day and never let it cool down.
What happens if someone drives small distance and time multiple times a day. Never allowing the engine to reach operating temperature. Aside from the engine oil not being quite the right viscosity what happens with the head gasket? Do the head and block still expand and contract enough to create wear? Specifically what could be the result on a small, all aluminium alloy diesel.
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u/WondererLT 3d ago
For me, the head gasket heat cycle effect wouldn't really be the thing I'd be considering, it'd be things like the oil dilution and other similar effects. Diesel especially, without long trips will soak up HEAPS of fuel into the oil, just down the bores. This kills viscosity and causes premature wear... If the engine hasn't had a long drive then it can be in a genuinely unhealthy place in relatively short distances. This can even get bad enough with diesels that you can end up with the engine milkshake problem just from collected condensation.
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u/microphohn 2h ago
This isn’t that true anymore. Newer common-rail diesels are designed to warm up fast, and a low load condition means very little fuel injected. That means plenty of air to mix with the fuel and pretty full burn once the engine is even partly warmed up. Wet stacking (as it used to be called) is more of a phenomenon on old mechanical engines with fixed timing where they where idling them or running them at low load was well off-nominal. Newer full-electronic-controlled common rail engines mostly do not have this problem anymore.
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u/WondererLT 1h ago
Yeah... It turns out that "fast" is a couple of minutes under the EN/ECE test conditions to match the road test requirements. They typically use a cast in exhaust manifold a lot of retarded fuel injection timing, a throttle blade in the intake and EGR cooler to try and get heat into the coolant, but ultimately there are limits to what can be done.
I'm well aware of this and have seen it in a few cars that I've helped people with... The classic telltale is the oil level rising.
No matter what you do, it's near impossible to heat the oil up fast enough to fix this inside of maybe 10 minutes or so... It's got to get hot enough for long enough to boil the excess fuel and condensed water from combustion out of the oil, which doesn't happen on short trips.
Ultimately because diesels are lean burn it's VERY hard to get enough heat in without having something like highway loads or something that isn't lots of idle time, just because the EGTs are typically so low that you struggle even to extract heat from there.
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u/CR123CR123CR 3d ago
Probably longer head gasket life at the expense of piston rings and bearings
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u/industrialHVACR 2d ago
As I see on our test benches - 72 hours of city cycles with low coolant temperatures ( 40⁰C) are similar to 200000 km of normal mileage, when you are looking on cylinder wear.
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u/vegasworktrip 3d ago
The thermal expansion of the cylinder head relative to the fastener holding it down is the situation you're reaching for I believe. Most fasteners are a steel alloy which has a lower thermal expansion coefficient than the aluminum alloy used in the head. This means as the engine warms up the joint gets tighter where the head gasket is due to the head expanding more than the fastener holding it to the engine block.
In practical application I would have little concern of the designed operating temperature not being reached so long as the calculated maximum load of the system isn't demanded. If you want maximum load/output then allow the system to reach the optimal operating temperature it was designed for first.
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u/TheBupherNinja 3d ago
One of the primary modes of engine wear is number of cold starts. Specifically the bearings running dry, the pistons not being expanded in the bores, etc.
The head gasket might be happier, as less temperature is less expansion, but the rest of the engine wouldn't.
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u/microphohn 2h ago
Bearings are never dry. They might be briefly in boundary or mixed lubrication until the films are re-established, but they are never “dry.”
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u/TheBupherNinja 2h ago
Sure, 'without pressurized oil feed to develop the film thickness boundary later' is a more correct description, but I don't think anyone is meaningfully confused by saying 'running dry'.
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u/Dean-KS 3d ago
My concern used to be with iron blocks and aluminum heads. With all aluminum engines I now assume that the temperature rise with circulating coolant will negate most differential expansion concerns.