Hey everyone! I was just watching a CarCareNut video and in it he states that if a car recommends premium gasoline YOU MUST use premium gasoline or else the piston could either melt or break in half. My question is simply is this true?
It depends on how good the engine management system is at detecting the onset of preignition and stopping it.
The context of that statement was in turbocharged engines, fyi.
turbochargers add approximately 10 - 15 psi of "boost" (pressure) to the intake charge. That's close to 1 atmosphere. (naturally aspirated engines draw in ambient air, which already has 1 atm of pressure). So roughly speaking, you are doubling the combustion pressure inside the cylinder. This also greatly increases the temperature of the mixture as it gets compressed by the cylinder.
There's a bunch of stuff that happens next, that has to do with flame front propagation, ignition retarding, gasoline stability, pressure waves, secondary compression, valve timing, turbulence, EGT, chamber shape, charge density and stratification etc. It gets complicated so quickly it can make your head spin, and engineers need to be very scientific about it when optimizing an engine. Information about it is available on the web if you want to learn about it.
All this to say, turbocharged engines are much more prone to knock, and there's only so much the computer can do to compensate. So yes, use the recommended fuel grade.
the piston could either melt or break in half. My question is simply is this true?
if you really pushed the engine, and ignored the knocking, and there were no computer safeguards, then yes
My question is simply is this true?
It depends.
A well built turbocharged engine that doesn't explicitly require premium fuel can run on regular fuel very well. The two examples that pop to mind are the European late model year PureTech and BoosterJet - I am seeing lots of the latter reaching over 200k miles.
A lot of engine builders have learned how to deal with turbos (and GDi), on the Euro market in particular car manufactures are fully relaying on a turbocharger to make their small displacement ecological engines drivable - in some cases (like G3LA (MPi) --> G3LC (T-GDi)) almost doubling the output (67 hp / 95 nm -> 125 hp / 172nm) while retaining a somewhat decent level of reliability.
In contrast, a poorly built turbocharged engine may struggle with uncontrolled low speed pre ignition (LSPI) and in the case you experience such symptoms you should use premium fuel. (This applies to some Ford Mazda derived 2.0L EcoBoost engines, and to the worst of them all, the GM-Opel A-series)
Potentially. Premium higher octane fuel won’t, pre-combust as regular lower octane fuel might. If fuel combusts before expected, it could create knock. Hence potentially damaging the engine, ie the piston.
just watching a CarCareNut video and in it he states that if a car recommends premium gasoline YOU MUST use premium
The only thing I've heard him say is 87 octane is more volatile than 91 octane. Don't recall seeing a video like you've referred too.. I found this one from 2 months ago where he clearly states "only in high compression, high performance engines"
@TheRealFarbs Thanks for posting this video as I hadn't seen this yet. So when fueling a modern car with a turbo, stick to the manufacturers required octane rating or you could ruin your engine.
he states that if a car recommends premium gasoline YOU MUST use premium gasoline or else the piston could either melt or break in half.
You probably misheard him, or misunderstood him. Pistons don't melt under normal circumstances that the engine was designed for. The only thing I'm aware of is pinging, or knocking, that's caused by detonation (the volatility of the fuel means the piston is still on the intake stroke when it detonates, you want computer-controlled ignition NOT detonation). This can seriously damage the engine, or outright destroy it, especially if you have an old school carburetor and the engine is purely mechanical. The head of the piston is the weak part. You could easily bend or outright destroy the piston head using the wrong octane fuel, but not the entire connecting rod/piston assembly. You said yourself that premium fuel is recommended. You didn't say it's required. If it's a more volatile fuel (such as regular gas from the gas station), you will get less boost out of the turbo before detonation occurs. Detonation will eventually kill your car if left unchecked.
Pistons aren't just made out of one material these days. The piston head is aluminum, and the connecting rod is made of steel. There's around 1,000⁰F difference between aluminum and steel's melting points. I know that one, because when I attempted to plate a piston and connecting rod, the chemical engineers told me I couldn't do it all (I used to work at a plating company). I would have to break the thing down and anodize it, etc.
See at 17:15:
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https://carkiller.com/scottykilmer/postid/294883/
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CarCareNut is a good mechanic and generally knows what he is talking about.
pistons can indeed melt. combustion temperatures in the combustion chamber rise extremely high on the compression stroke, and the pistons are pretty far from any coolant.
Coolant jackets encase every cylinder. There's only a couple inches between the coolant and the piston walls. I'm pretty sure the coolant runs in 4s around every combustion chamber. Neglecting the combustion chamber, there's probably an inch or so between that and the coolant. The cylinder walls are designed to whisk the heat from combustion and into the coolant.
but pistons aren't in direct contact with the walls. The only way for the heat to conduct is through the thin rings which are very, very poor conductors. It would be like pumping a swimming pool through a drinking straw.
I guess I misspoke, it should be under "normal operating conditions", for that particular engine. I was assuming operating conditions were ignition based, not detonation based. Sorry.
using the wrong fuel can lead to abnormal conditions.
I know that, lol. I'll edit my response.
I said that in my initial post, lol. As long as you're igniting it via computer and not detonating it when it's still on the intake stroke, there's no harm done. You don't want it to detonate (that produces a double whammy, the piston is still trying to move upward, confining the burning mixture into an ever-smaller state, that further increases the temperature as it burns, which is the exact opposite of what you're trying to do, lol). The detonation part was covered has seriously damaging, or outright destroying the engine. I edited it anyways, to be clearer in my response.
you don't even want secondary ignitions on the compression or exhaust strokes. Any kind of secondary ignition from hot spots is bad as the pressure wave fronts collide. You want a nice smooth burn from one end to the other.
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The question seems pretty clear cut. Can the wrong fuel cause piston damage. The answer is yes.
I know that as well.
You might have glazed over the premium fuel recommended part, not that it's required, lol. If premium were required, you would get engine knock, if you use the wrong fuel. Per the owner's manual, it says recommended, not required.
and I'm saying that since it's a turbo-charged engine, you should probably listen to the recommendation. Much less margin for error.
In a high-compression naturally aspirated engine, the conditions are a lot more controlled. You don't have runaway exhaust gas temperatures, and the timing is well within the range of retard compensation.