Can squish clearance and boost hydraulic an engine?

[RedLeg]

Hello everyone,
I am new to the forum although I have been lurking by finding lots of answers here through Google.
In A. Graham Bell's "Forced Induction Performance Tuning" Bell notes that reduced squish clearance can cause an engine to hydraulic.
I have reviewed my basic thermo books but can't figure out how to calculate when a fuel/air/water mixture will be "solid."
Specifically, I would like to determine at what boost and CR this could occur.
Dan

 

[BrianPetersen]

Under any reasonable ambient conditions, air is above its critical temperature (by a lot). This is the temperature above which it can only exist as a gas, no matter the pressure. Most ordinary fuels are not in that condition - they can exist as a liquid or a gas.

From the stoichiometric air/fuel ratio and the (known) densities, you can figure out how much space the fuel takes up when it is in liquid state. For gasoline, the fuel in liquid state takes up approx 1/10,000 the volume of the air at normal atmospheric conditions. If the compression ratio is 10:1 then the fuel in liquid state takes up 1/1000 the volume of the air. No "reasonable" supercharging pressure and "reasonable" compression ratio and "reasonable" air/fuel ratio could ever cause a gasoline/air mixture to hydraulic-lock an engine.

That's not necessarily true of all fuels, though. I believe the statement could be true of nitromethane in drag-racing applications. Drag racing engines are heavily supercharged and the air/fuel ratio is extreme because nitromethane carries along a lot of its own oxygen.

If the fuel takes up a substantial amount of the compression space, keep in mind that even though it might not actually be compressed "solid", the pressure may go high enough that, from the point of view of bending con-rods and blowing head-gaskets, it might as well be solid.

 

[izzmus]

I've heard the legend of hydraulic lock in a Top Fuel engine and I refuse the buy it. While they do run air/fuel ratios close to 1:1, the boost levels aren't that extreme. Maybe a few misfire cycles can cause a hydraulic event.

Now, those 200+psi boost tractor pulling Diesels that run hefty amounts of water injection may be candidates for hydraulic lock...

 

[patprimmer]

You can believe it. If you get it wrong with a positive displacment blower at 40psi boost, 11:1 and a high %age of nitro methane and the mixture to rich to avoid detonation and some leakage past the nozzles, it will hydraulic lock.

I never heard of a top alcohol engine locking though. You need a fair swag of nitro in it to risk locking it.

It cannot see how itwill not lock because of squish unless you do something real stupid with piston or deck surface shape.

The whole idea of squish is to squeeze almost all the gas out. It does this progressively as the piston comes up and the gas over the piston can escape due to any pressure difference. If dynamic effect caused a temporary pressure spike over the piston, the gas would flow faster well before the air solidified.

Also pistons rock in the bore so the tightest side would slope toward the chamber due to rock. This would assist any liquid out of that space.

Also a problem can only occur at very near TDC and at this point the piston is travelling real slow, relatively that is.

Regards
Pat
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[RedLeg]

Thanks - great responses.

It makes a lot more sense that the cylinder pressure could be too high than the mixture locking up.

There must be a tipping point where boost pressure and compression overcome the force of the crank.

Do you know any calculators that determine the cylinder pressure generated on the compression stroke?

The intake air charge temp must play a big factor.

Dan

 

[evelrod]

I'm probably not understanding what Pat said in relation to "squish". In my experience increasing the squish area in a given engine, disregarding any increase in CR, increases the mixture temp thereby aiding the vaporization of the fuel and by "squishing" the mix across the combustion chamber radically tumbles the mix aiding in a more complete combustion. At least that's what I have found in the small I-4 engines I've been working on lately. Even at 16:1 CR and piston to head clearance of as little as 0.010" there is no possibility of 'locking' a cylinder. Not to say other failures can and, often do, occur. I generally don't go tighter than ~0.25" as less than that leaves no room for error.

As to the big drag engines? I have so little experience in anything approaching 'modern' that I can only give the one example that I believe to be true... Test of a Keith Black engine, I am told by the dyno operator that it hydrauliced a cylinder and then blew the blower off when the throttle was inadvertently opened full and abruptly closed. That's second hand, but I trust his statement to be true.

Rod

 

[patprimmer]

Rod

I posted late at night got a bit lost and phrased some of it poorly and some of my post is not what I meant to say.

Below is a fix.

Brackets () indicate what should be removed and all additions are in caps to indicate changes, not to yell

You can believe it. If you get it wrong with a positive displacement blower at 40psi boost, 11:1 and a high %age of nitro methane and the mixture SET SO rich AS to avoid detonation (and) OR some leakage past the nozzles, it will hydraulic lock.

I never heard of a top alcohol engine locking though. You need a fair swag of nitro in it to risk locking it.

It cannot see how it will (not) lock because of squish unless you do something real stupid with piston or deck surface shape.

The whole idea of squish is to squeeze almost all the gas out. It does this progressively as the piston comes up and the gas over the piston can escape due to any pressure difference. If dynamic effect caused a temporary pressure spike over the piston, the gas would flow faster well before the air CAN (solidified) turn to liquid phase.

Also pistons rock in the bore so the tightest side would slope toward the chamber due to rock. This would assist any GAS FLOW OR liquid FUEL out of that space.

Also a problem can only occur at very near TDC and at this point the piston is traveling real slow, relatively that is SO THE AIR HAS TIME TO ESCAPE BEFORE THE PRESSURE GETS THAT HIGH.

ALSO IF THE PRESSURE GOT HIGH ENOUGH TO LIQUEFY THE CHARGE, YOU IT WOULD ALSO GET HOT ENOUGH TO DETONATE BEFORE THAT HAPPENED.

I DO BELIEVE THAT WITH NITRO METHANE, LIQUID FUEL IN THE CHAMBER CAN INCREASE COMPRESSION SUBSTANTIALLY AND CAUSE DETONATION

Regards
Pat
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[patprimmer]

The "not" being left in the third paragraph really was a major stuff up on my part. I can only presume I left it there by accident as I rephrased while composing.

Regards
Pat
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[SomptingGuy]

Wasn't "squish" a game perfected by the late Ronnie Barker?

- Steve

 

[Metalguy]

Way back in 1971 when I had a funny car (AA/FC), the pistons were something like 1/8+" "down in the hole" at TDC. Squeeze was only ~6.5 to 1. Dunno what they do these days.

BTW, liquids above their critical temps. do become gases, but their density can be the same as the liquid--they ain't gonna compress like a gas at any non-explosive pressures.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[evelrod]

Okay, Pat's not the only one making mistakes. I posted that I generally use 0.25" clearance....No. That should be 0.025"! There is a great deal of study and testing done to determine the 'sweet zone'. Just making it tighter is not always the answer. There was a very good article on 'swirl', 'tumble' and, 'quench' published by Suzuki a few years ago. Anyone remember?

By the way, squish is just what the proletariat refers to the quench area. I'll not say squish again. Also, it's hard for non race engine oriented engineers to "see" a 0.010" stretch in a rod/piston assembly...brother, it happens. Often a lot more, too.

I also dislike reference to a Panhard rod as a 'track bar', too. Plus a few more but that probably belongs in the
engineering language forum.

 

[patprimmer]

I have always thought quench came from the gap between the piston and head getting close enough to cool the charge enough to quench any flame in the area.

I always thought of squish as the piston getting close enough to the head to squish out virtually all the charge from the area and set up swirl or more likely tumble in the chamber to spread the flame kernel.

I presumed the confusion came from both functions resulting from tight piston to head clearance in the areas a long way from the plug.

This is supposition on my part rather than a study of credible sources.

Regards
Pat
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[evelrod]

Picture a full hemi combustion chamber. I think it was Smokey that said he liked to set up his clearance so that the pistons made a "witness" mark in the quench area on the cylinder head...I'm not that brave. According to all that I have read and most of what I have experienced, the tighter the quench area clearance, the bigger the "squish", etc. (oh crap, I said it again).

I can get good power with no detonation with my wedge head at over 16:1 CR whereas I can only get about 13:1 CR on the hemi to work on the available (to me) 110 octane fuel. I mean work to optimum power for the displacement.

Rod

 

[patprimmer]

All the engines I ever had that performed better than expected had witness marks on the pistons at least after considerable miles and increased piston to bore through wear and increased bearing clearance through wear.

I have had witness marks on engines with 0.038" piston to head. Some of that 0.038" may have been due to differential expanion as the piston is aluminium and athe piston, rod and crank are all hotter than the block, butsome was surely due to slack in clearances being pushed in the same direction and due to stretch and piston rocking.

Regards
Pat
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[scottspeed]

Working with high performance race engines, I have seen this a few times before. The engine doesnt exactly hydraulic and stop dead, and sometimes it will not be noticed until the engine is stripped down.
The most common form is on restricted turbo charged engines. When particular rich fuel mixtures are run it seems possible that the fuel droplets do not actually atomise and can lead to hydraulicing between the piston and the head area, resulting in bent connecting rods. From this i have seen a 300hp restricted wrc type engine destroy (bend) conrods that are fine in 1000hp drag cars.
The matter is made worse if water injection is also used.
If you find build specs for WRC engines, or high output hill climb type turbo engines, alot most of them will have either no squish area at all, or the piston will be about 2mm away from the squish pads.

 

[evelrod]

Scott, I've only done one turbo engine (can't count my'64 Corvair Spyder, I was too young). A 2.3 Turbocoupe to 351hp @ ~5800rpm on the JBA dyno, 295 at the wheels. It had a very small quench area almost nil. Can I assume that because the intake mixture is under a very high boost that the fuel is better atomized and mixed as it is injected around the inlet valve? This was a '83 Bird in the late 80's on pump gas. I pretty much went by the "instruction sheet" from JBA/SVO at the time. I had almost no experience with supercharged engines...Still don't!

Rod

 

[yoshimitsuspeed]

To the people who say they have seen this. How do you know it wasn't preignition or detonation or something that could also create extreme cyl pressures?
Or even if it really did hydraulic is it possible to know it wasn't a mechanical failure like an injector sticking open or something?

The theory about high ratio fuels like Nitro sounds possibly plausible but I find it hard to believe in most situations.
I have known many people and groups running high boost and making big power, many using water and alcohol injection and have never heard of this being suspected for an engine failure.

 

[patprimmer]

I have run 40# boost with a roots blower on a SBC on straight methanol at about 5:1 a:f. Never damaged a piston or rod or anything else in any way that could be seen as hydraulic lock. We have broken rod bolts and burned holes in pistons, but they are obviously not from hydraulic lock.

I certainly have seen hydraulic lock with water injection when things go wrong, like it siphons a cylinder full of water when parked or the pump switch fails in the on position.

Regards
Pat
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