The final Dope on 4 stroke engine wear.

Describe the difference between the type and location of wear of these 3 scenarios:

  1. When your engine is running no-load RPM’s (revving in neutral for instance)

  2. When the engine is in the midst of downshifting to slow at a light (no throttle)

  3. When the engine is at the same RPM’s but with full throttle.

(all these scenarios are with everything else equal, such as time for change in RPM to give the oil the same chance to respond to the change in RPM, ect.)

***Are these answers different for diesel engines and two-strokes?


By the way, I never downshift to slow down, kinda stupid to wear out clutch when brake pads are so easy to replace. Just thought I would clear up any assumptions.

I forgot my most important scenario:

  1. When the car is in too tall/big of a gear and you step on the gas (5th gear at 30 mph).

So, add this “too-tall-gear” scenario to the list above. thanks.

Is this too tough of a question for the almighty Dopers???!!! If you don’t want to answer the first three scenarios, please answer the last one!

For #3, I’d go with more wear on the rings since the pressure in the cylinder is high under heavy load. Just a WAG though.

High revs mean high demands on pushrods (if so equipped), belts and gaskets as pressure builds.

Also, valve spring wear is an issue with high revs.

Fuel, you could just about write a book on the questions you asked.

I will take a whack at it Tuesday (the next day I have any amount of free time available), if no one else answers your questions before then. It’s a topic that interests me, too.

Philster is on the right track with the cylinder pressure. It all has to do with where in the cycle the cylinder pressure peaks.

Certainly, it’s true that marine engines last nowhere near as long as automotive road engines - and the reason is the constant work loads experienced by a marine engine while driving a boat - after all, you’re pushing a consistent truckload of water out of the way as you move along - compared to a road engine which often gets to “coast” or roll along at “low workload” levels.

That being said, the best engine I’ve personally ever owned, in terms of smoothness and performance, was a Suzuki GSXR-750.

When I was breaking it in, it was suggested to me that each day, that I find a long constant hill and drive up it in top gear at low revs and slowly apply wide open throttle. And the theory? It would really bed in the rings etc.

Well, not long after I broke the engine in, I started riding it 100 miles a day (50 miles each way) from home to university and back - which obviously is a fantastic way for an engine to live - compared to stop go city driving.

Man, that was one sweet revving, clean burning engine.

I mean, you could write a book, or you could generalize and say that the more stress you add, the more wear:
heavy load(means accelerating from low rev, not towing per se)- pressure and surge it heat…rings, gaskets, turbos all under added stress.

hi revs - anything moving and wearable is under stress. I think valve springs first. Tough little component…very important little components.

Downshifting? Big deal…little issue if done right.

Neutral? Least stress.

There are tons of exceptions, and what if scenarios.

Generally, hi revs and heavy acceleration take the biggest tolls. Think of a race car issues. What tears them up after their short lives? Idling in the pits or running at 8,000 rpms for hours?

Downshifting for engine braking is often not the reason with a two-stroke. You want to keep the revs up for the exit to the corner, not to slow down.

Thats about all I have to add to this. If Kevin Cameron were on the boards, he could clear this whole thing up, in language we could all understand in 1500 words or less. The guy is a genius!

Fagjunk Theology: Not just for sodomite propagandists anymore.

So, when flooring it in first gear up a hill, the cylinder and rings and everything else is under most pressure, heat and abrasive. This much is obvious to me. But does this mean that the compression ratio changes if you drive the car in the wrong gear, or is there a difference between CR and cylinder pressure at ignition? I guess I could stand to understand CR more thoroughly…

Again, if this question is too in depth, feel free to just answer one scenario. thanks.

Compression ratio is a constant based on cylinder bore and stroke. Cylinder pressure on the other hand is not. This is due to the fact that the engine is starved for air unless the throttle is wide open. (intake is in a vacuum)
Here is an interesting little experiment which may answer some of your questions.
Take a compression tester (screw in kind) and install it on one cylinder of your car. Disable the ignition and crank the engine until the higest reading is achieved.
Remove the comppression tester and take the shrader valve out of the end of the hose. Reinstall the tester. Reconnect the ignition and start the car with the compression tester installed.

Typically you will see, say about, 160 PSI cranking and about 50 PSI at idle. The difference is due to the fact that an engine is starved for air at idle.

Compression ratio never changes, unless you use a wrench.

There is a cylinder pressure peak that happens inside the cylinder after the ignition event. If crank is turning relatively slowly (low RPM) the crank hasn’t turned through as many degrees of rotation before the cylinder pressure peaks, causing it to happen closer to top dead center. The closer you get to top dead center, the less mechanical advantage you have. You can imagine how bad it would bad if this peak were to happen at TDC. When you “lug” the engine (high power at low RPM) you are moving this peak towards TDC. You are beating on the bearings, right at the time when they have less oil pressure due to the low RPM. You are also beating on the piston rings, wrist pins, etc. When you pull high power at high RPMs, the faster spinning crank has moved through more degrees of rotation in the time it takes the peak to develop. The mechanical advantage is better, more of the pressure is being turned into power, and your engine isn’t getting beat up as badly.

When you are cruising at 2500 rpm and encounter a hill, the engine RPM will drop. You give it a little more throttle to make up for it, allowing more of the fuel/air mixture to flow into the engine. With a constant RPM, to make more power you have to increase the cylinder pressure. Now, you have more cylinder pressure combined with the peak happening closer to TDC.

That is the very short version of why it is bad to lug your engine.

Well, in a word no.
God I was hoping to avoid this, but here goes.
Your explaination would work if the engine we were talking about had fixed ignition timing. Automobile engines have variable timing what adjusts with engine speed along with other factors.
OK, here is the deal. For best preformance, you want the maximum cylinder pressure to occur at about 5-10 degrees after the piston has passed the top of its stoke (design dependent). This is referred to as ATDC (after top dead center). In other words you want the expanding gasses to exert the most pressure on the piston when the crankshaft has turned about 5-10 degrees past the top of the pistons stroke.
Now it takes some time to burn a cylinder full of fuel. About 3ms.
At 1000 Rpm 3ms is about 15 degrees of crankshaft rotation. This is why on most non-emission control engines the base timing is between 5-10 BTDC. (10 Degrees Before top dead center, plus 15 degrees = 5 degrees ATDC)
When you increase the engine speed to 2000 RPM the 3ms is constant, but the crank is turning twice as fast. Now the spark event has to occur about 30 crankshaft degrees before 5 degrees ATDC, or about 25 degrees BTDC. (-5 +30 = 25)
There comes a time when an engine will not benefit from any more advance. The total advance available varies from system to system, I know of one ignition system that will advance up to 70 degrees BTDC.

The rule about not lugging engines has more to do with piston ping (engine knock) and oil pressure than anything else IMHO. By going to wide open throttle at low RPM extra fuel and oxygen are admitted into the cylinder. This makes the explosion larger than normal, and more prone to autoignition. Severe engine knock can damage engines.

Joey G, superb technical answer. Thanks to Rick too.

Rick, so the engine is starving for air at idle? Does that mean that the air to fuel mixture is too low or that they are both too low? What exactly do you mean by “starving”?

What type of ignition systems do for instance a Suzuki gsxr or Yamaha R have? They rev really high, how do they deal with this BTDC angle degrees?

What parts are “knocking” together, by the way? Also, isn’t there a computer that makes sure that not too much fuel and air get in the cylinder?

So, basically when the engine is turning slowly and lots of power is exerted on it, the explosions happen when the piston is not anywhere near ATCD, causing knocking?

I was talking about an engine that had fixed ignition timing, but I guess I should have said that. That’s the problem with giving in and posting a short version.

The basic answer is correct, though. Even with variable ignition timing, you have higher cylinder pressure at a time when you have less oil pressure.

**Now we are getting somewhere. What is the knock, and what happens inside the cylinder that causes it?

I just got the hint from the last post that RPM’s determine oil pressure? Also, how much of a lag is there between rising RPM’s and rising oil pressure on a high performance engine such as the Suzuki or a Formula Firebird?

Oh, and I forgot this one. Where in the cycle do you see it?

Check out , the image about halfway down the page titled “Cylinder pressure profiles”. Note where the rise in pressure happens, and compare it to what I said:

OK, last post before I go to bed tonight. let’s see if I can answer a few questions first.

From Fuel

OK I’m gonna explain this on the engines I teach on (Volvo) cause it’s late and I’m tired. The engine displaces 2.5L over 5 cylinders. each cylinder is 0.5L. If I block the thottle open and turn the engine over two revolutions of the crankshaft I will have moved 2.5 liters of air though the engine. (each cylinder will have gone through the intake/compression/combustion/exhaust stroke. But we don’t run our engines with the throttle wide open all the time do we? At idle the throttle is almost closed. Just a tiny crack of a gap around the edge. Now remember that every two revolutions of the crank the engine would like to pull 2.5 liters of air though. at 800RPM that would be 400 *2.5L = 1,000 Liters of air in a minute. To get an idea of how much that is, a 1,000 Liters is about 250 gallons. Which is a little over four 55 gallon drums of air in 1 minute. Now remember the throttle? Do you really think you can get almost 250 gallons of air though that little crack in 1 minute? or course you can’t. That is why the intake manifold runs in a vacuum unless the throttle is wide open (I’m not talking turbos, or superchargers here). The engine wants more air than the throttle will allow it to have. This is why I said the engine is starved for air. The air is what throttles the engine. The mixture is correct for the amount of air that is in the cylinder. As the air flow increases so does the gas flow maintaining the correct mixture. (yeah I know there needs to be enrichment for acceleration, I’m trying to make this simple)

Again from Fuel

I’m a car guy, I really have no clue as the the design specs for a gsxr or R. I would assume that it is some type of computer controlled system based on a RPM sensor on the crankshaft. I’m not sure if I understand your question aobut BTDC angle degrees. I think what you are asking is how do the designers determine how much advance to use? I believe its done though calculation backed up with dyno time. don’t ask me the calcs, I have no clue.

More from Fuel

Engine knock can be one of several things. first off if there is a hot spot in the cylinder ( a chunk of carbon glowing red perhaps) the fuel air mixture can be pre-ignited causing the mixture to burn too soon and cause all kinds of problems. For example if the mixture were to ignite too soon, it might reach maximum pressure before the piston had reached the top of its stroke. This type of knock can cause damage to the engine if it is bad enough. The second type of knock is when too low of an octane fuel is used, and it burns un-evenly causing two (or more) flame fronts in the cylinder to colide with each other. This causes a mini sonic boom that sounds like marbles being rattled in a can. This is called piston ping or ping. This is probably not a problem if it is not too bad.
yes in today’s engines computers control the fuel mixture, ignition timing, and on some engines even the throttle opening.

Still more from Fuel

Lets get some terms straight here. when the piston is at the exact top of its travel that is called Top Dead Center (TDC) If the piston has not yet reached TDC that is called Before Top Dead Center (BTDC). If the piston has passed TDC it is called After Top Dead Center (ATDC) Go look at the link that Joey provided scroll down to the picture called “normal combustion cycle” The piston on the left (ignition) is BTDC the one labeled TDC is TDC, and the rest are ATDC. You want the power to be applied to the piston ATDC

From Joey G

Well yeah, you should have. Since the OP mentioned downshifting, and I have never seen an airplane that could be downshifted, colored me lost when I read your post. :confused:

Again from Joey G

Probably a non issue with most modern and well maintained engines (automotive, motorcycle) sure the oil pressure is less than max, but it should be more than adequate, or the designers didn’t do their job right.

Once again from Joey G

See my answer above. Something in the back of my mind keeps naging me that there is a third cause of ignition knock, but I’m too tired to remember it now. If I can find it I will post more.

Once again from Joey G

I assume you mean the knock? pre or self ignition would be before TDC. Look at the site you linked. See the orange curve? Look at it to the left of TDC. See how it is above the green line? This excess pressure BTDC can cause real problems. Now picture if the mixture had ignited before the plug fired. (mentaly move the orange curve to the left to where the top of the curve is BTDC. this could be real bad real quick. Ping is after the spark has fired. could be BTDC or just ATDC

One last from Fuel

Oil pumps on most engines are mechanical off the crankshaft or camshaft. There is a lag, but it is probably not worth talking about.

It’s late and this has turned into the post from hell, so I’m off to bed.

Indeed, all of the above posts are excellent.

Just as a bit of extra reference - a quick compendium on “ignition timing” in the context of super high revving engines like Formula One engines…

I had the good fortune in the early 90’s to speak to a Mr Peter Morgan (I’m really, really sure that was his name - apologies if I got it slightly wrong) and the gentleman was one half of the fabled Ilmor racing engine company emanating out of Britain - hence the “mor” in the “Ilmor” name.

At the time, Ilmor was owned by Chevrolet, and they (Ilmor) were making most of their money by selling Chevrolet badged engines to the Indy Car circuit. However, they were also making V10 hi-octane “gasoline” engines for Formula One - but I hasten to add that I use the term “gasoline” very loosely because the fuel used in Formula One is anything but normal pump gasoline.

Anyways, I asked some general questions at first, which then got more and more specific as Peter was gracious enough to keep answering them. My first question revolved around the design differences between the Indy Car engines and the Formula One engines - and his answer was most interesting. Bear in mind, I’m gonna try and repeat his answers as close to possible as verbatim as I can remember…

PM: “Disregarding the obvious, such as turbos and the lower compression ratios which turbos require, the single greatest design hurdle we’re consistently facing at the moment is how to guage the variable degrees of flame front propagation which the Formula One engines are experiencing compared to the ethanol engines in Indy…”

Me: “What the bloody hell is flame front propagation?”

PM: “Well, in the F1 engine, depending on the batch of fuel we get, it burns at different rates - it all has the same octane but the amount of Boron the ELF people are adding at the moment is variable from week to week based on what they’re discovering with the work they’re doing for the Renault people…”

(Note: at the time, the Renault F1 engine was the undisputed heavy weight hp per cc champeen of the world… for a normally aspirated engine)

Me: “The ELF people are stuffing you around are they?”

PM: “Oh for sure… they’ve got about 150 scientists working full time in France at the moment on absolutely nothing but Formula One racing fuel… almost every single aspect of that research is going into keeping the Renault engine at the top of the heap, but they sell to us as well as a 2nd teir engine supplier… the problem for us is that we can never tell when we’re gonna get a super batch of fuel… in any week the total horespower might jump as much as 50hp if it’s really wicked stuff…”

Me: “Far out… so what are they putting in it to make it so wicked?”

PM: “Well, it’s mostly pure Toluene but it has all sorts of wicked additives like Boron and stuff like that. The flame front propagation is a measure of how quickly the fuel burns from the centre of the spark to the cylinder wall. We’ve had to invent a crystal based solenoid which resonates with engine ping - and our on board CPU’s constantly push the ignition advance as close as possible to TDC before the solenoids detect the ping resonant sound waves. When the solenoid detects the signature sound wave, it notifies the CPU in the engine management system and the CPU moves the ignition further away from TDC…”

Me: “So how does this compare to the Indy based engines?”

PM: “Ahhh… you see, the ethanol provided in Indy is a control fuel - it’s the same fuel for everyone - and the nature of ethanol is that it’s very easy to distill to a very pure state - which means that the knowledge base for ignition timings and flame front propagation is a known constant. That’s why the ignition curves for our Indy engines are burnt into our CMOS chips and we never have to adjust them - ever. But in our F1 engines - man - the sky’s the limit. Every week there’s another tweek we have to do just to stay on the pace. It’s amazingly hard work.”

Me: “So what are the main differences in engine RPM?”

PM: “Our Indy engine tends to max out at around 12,500 rpm in terms of maximum horsepower. We can get more, but it’s a case of diminishing returns when we factor in the fuel consumption aspects… but in F1, this is our 2nd year of runnning pneumatic compressed valve actuation and we’re currently hovering around 16,800 in race trim, with the option for the driver to go to 17,500 for short bursts… but I think we’ll be seeing 18,000 rpm in all of the race engines within a few years without any doubts…”

Me: “Far out! The fuel burn in those cylinders must be like a constant oxy-acetylene torch!”

PM: “It seems that way, certainly. But we tune our exhaust systems to scavange the exhaust really well - our sensors show only about 5% unburnt fuel on valve overlap overruns… the main impediment is guaging the inlet manifold shock waves at those sort of rpm’s - it helps that the air intakes above the driver’s helmets kind of forces a ramjet effect, but still, we’re still doing research into our inlet trumpets to work out the best tuning lengths…”

Me: at this point I was in over my head and effusively thanked Peter Morgan for all his time and shook his hand profusely.

For your reference, not long after, the Ilmor company was bought by Mercedes-Benz and all the Indy cars which were running Chevy badges swapped to Mercedes badges. At around that time, also, McLaren cut a deal with Ilmor and soon thereafter went on to win a number of World Championships with Mika Hakkinen at the wheel.

fuel, exactly what are you trying to find out here? maybe if you were a little clearer with maybe the context of the question, or the arguement you were having with your friend that spurred this question, we would be able to tell you exactly what you’re looking for