cold start vs warm start for cars

It is my understanding that the majority of wear to a car’s engine occurs when you perform a cold start. When the oil is in the pan and not coating the cylinders.

Is the wear associated with a warm start, then, of trivial wear?

I once read an article where it said the most damage does indeed occur at startup, but not for the reason you have mentioned.

At startup, and for a few minutes afterwards, the cylinder walls are cold. Some of the fuel condenses onto the cylinder walls and runs down into the oil pan via gravity. In other words, every time you start a cold engine, you are pouring a little bit of gas into your oil. Adding gas to oil is not a good thing; gas is not a good lubricant, and it decreases the viscosity of the oil. And the oil filter can’t do anything about it.

This also explains why a lot of short trips are hard on a car. When the car runs for a while that gas eventually gets removed from the oil. With a lot of short trips the engine never warms up enough for this to happen and it starts to accumulate.

Right. But I’m not sure how efficient the PVC system really is, even after the engine is run awhile. I’m betting some gas accumulates in the oil regardless of the presence of the PVC system.

One thing to keep in mind, though, is that with modern engines and modern oil pretty much all wear is trivial. Baring some sort of catastrophe like overheating or running it without oil, the engine will almost always outlast the car. Doing a lot of cold starts is shortening the life of your engine, but it’s a matter of the thing lasting maybe 375,000 miles instead of 400,000. Chances are your car is going to have long since gone to the scrapper because the transmission blew or the body was just falling apart.

Sort of. The big issue with cold starts on old carbureted cars was that the vaporized fuel would condense on the walls of the intake manifold before it reached the combustion chamber, and then from there it could often wind up running down into the cylinder and into the crankcase. That’s also why you needed a choke, because you had to add more fuel to compensate for the fuel that was condensing on the manifold.

On a modern fuel injected car, though, the injectors are pretty much right at the intake valve, so the fuel has far less of a chance to condense. That, along with generally better fuel metering, has mostly eliminated the fuel-in-oil problem.

An engine that has been idle has had all the oil drain away from the bearings, so the bearings have only a thin film of oil. At start up there is no dynamic lubrication. Worm or cold.

It doesn’t condense on the cylinder walls - it never evaporated in the first place.

Gasoline is a blend of many different hydrocarbons that evaporate over a range of temperatures. When you inject it into the intake port of a hot engine, all of it evaporates quite readily and burns completely; you only need to inject exactly the amount that’s going to burn.

When an engine is cold, only the lighter hydrocarbons evaporate, leaving the heavier ones to remain as a liquid. You have to inject extra fuel so that you can evaporate enough of the light hydrocarbons to form a combustible mixture, and the heavier hydrocarbons get drawn into the combustion chamber in liquid form and splatter against the cylinder walls. Then the piston moves upward, and those HC’s collect at the piston rings, diluting the tiny amount of oil that’s there, reducing its ability to lubricate the rings and prevent wear. If you subject the engine to heavy loads (i.e. stomp on the gas) during this period of time, you may cause metal-to-metal contact between the rings and the cylinder bore, causing wear.

This part is exactly right. You can’t burn liquid gas.
One thing no one has mentioned is condensation. For every gallon of fuel you burn you create a gallon of water.
When cold some of that water winds up in the oil. Short trips in cold weather you can wind up with a a water/oil emulsion (often visible on the inside of the oil cap as a milky residue.
Running the engine to full operating temp and keeping it there for 20 minutes or so will evaporate this water and any gas in the crankcase.

Probably true for most street cars. I have a car with a GM Racing engine in it (2.0L, 375HP). In order for any claims to be considered (note that this is a no-warranty engine as delivered), you have to show you had a pre-lubrication system installed. Mine has a Masterlube accumulator fitted, which puts 1 quart into the block within 5 seconds. Since my engine run/stop switch and starter button are two separate devices, it should always have the 5 seconds to fully unload.

It is probably worth doing for a high-performance street car. Of course, in that case you’d need to trade off any loss of factory warranty due to the modification vs. longer engine life post-warranty.

Looks like I was wrong. Thanks for this explanation.

I love the dope. :slight_smile:

A bit OT, but is this why some exhaust systems rust from the inside-out, especially when driven for short trips?

Yup. Short trips kill exhaust systems.

All true. It’s not clear to me how much wear this causes during a single cold-start, but many cold-starts without adequate run time can result in a lot of water condensing into the crankcase and compromising the oil properties. This is exacerbated by the fact that the rings and piston are sealing poorly during a cold start because they are not up to their proper run temperature, which means a lot of blowby is happening. But this blow-by quickly gets past the rings and into the crankcase airspace, so I don’t think it screws up the ring lubrication (much) with a single cold start.

Years ago I saw a series of videos made by a GM researcher who had installed a transparent plastic oilpan on an engine. The camera was watching that oilpan, and the changes in the oil’s visual quality during one/multiple cold starts was remarkable; it turned from a nice clean new oil into a chocolate milkshake. Yikes.

Engine oil has emulsifiers added to it which ensure that the water is dispersed throughout the oil instead of pooling at the bottom (you don’t want a big slug of pure water to pass through the lube system :eek:), and it also has detergents to take care of the corrosive combustion by-products that are included in the blow-by - but there are limits to what it can handle. This is why many car owner’s manuals tell you to change the oil more frequently if your driving routine includes many/frequent short trips or a lot of cold-weather driving. Some cars (my wife’s 2011 Honda CRV for example) use oil life modeling: the onboard computer pays attention to your driving history and estimates how much longer the oil will last before it needs to be changed. Cold starts and the amount of time you drive in between each one are included in that model.

Yes - not just the water, but the corrosive combustion by-products that go along with a cold start. The next time you’re out for a cold winter drive, look around for cars with a lot of white fog coming out of the tailpipe; that’s condensed water, and it’s a sign of a car that was recently started and still hasn’t warmed up the exhaust system completely. Once the exhaust system is warm, the water usually remains in vapor form all the way to the end of the tailpipe and manages to mix with the surrounding atmosphere without condensing. Also, the next time you’re stopped at a traffic light, watch the tailpipe of the car in front of you. If that car was recently started, then when the light turns green and they accelerate, you’ll often see a bunch of liquid water drool out of the exhaust.

City driving is tough on a car’s exhaust, especially in the winter. Low engine power output means low exhaust gas temperatures and low exhaust flow rates; your exhaust system will be very slow to warm up. Contrast this with highway driving, which jacks up exhaust flow rates and exhaust gas temperatures. Yeah, the higher ambient airflow over the outside of the exhaust system matters, but not as much as what’s happening inside the pipe. Want your exhaust system to last a long time? Don’t make a habit of short city drives during the winter.

I’ve discovered that using Full Synthetic motor oil reduces wear under cold start and all other conditions substantially. I had one car make it to 322,000 miles and still didn’t burn a drop of oil or show any signs of engine wear! It probably would have made it to the 500k mark and beyond had a drunk driver not t-boned me and totaled it.

One of my current vehicles, a 2006 Mazda3 I’ve had since new, is past the 190k mark and runs like new. My dad sold his '92 Accord last year with 384k miles on it (he also used full synethic oil).

Do the answers about cold starting and exhaust rot also apply to diesel cars?

Yes and no. There are three separate phenomena to consider:

Contamination of engine oil with unburned fuel
Here’s where diesel and gasoline engines differ substantially. During a cold start, a gasoline engine brings in a cold, combustible mixture that also drags in a bunch of liquid fuel, the latter being the problematic component that dilutes oil on the cylinder walls. OTOH, a diesel engine brings in a bunch of clean, cold air, compresses it until it’s searing hot, and then squirts in exactly the amount of fuel that’s going to burn. It starts to look even better when you check out this cross section of a diesel combustion chamber. You can see that the crown of the piston (gray) has a kind of bowl-shaped cutout, and the fuel is injected as an array of plumes (orange) contained entirely within that bowl. Unlike a gasoline injector, a diesel injector (marooon) operates at extremely high pressure (upwards of 20,000 psi for a modern on-road engine) and is injecting into a highly compressed body of superheated air; the result is rapid atomization into a mist of extremely fine droplets, which evaporate very quickly. Bottom line? The cylinder walls of a diesel engine will never be visited by liquid diesel fuel.

Exhaust rot
Yep, pretty much the same as for a gasoline engine, except that a diesel is likely to be more sensitive to power demands. Diesel engines idle extremely efficiently for several reasons, which means they don’t dump much heat out with the exhaust gas. Moreover, they run very lean (lots of excess air), so the exhaust under idle conditions can be very, very cool. Result? If a diesel engine spends a long time at idle, the exhaust system may not get very warm at all.

Contamination of engine oil with moisture
This happens for diesel engines too. Like gasoline, diesel fuel is a hydrocarbon, and it produces water in about the same proportions as gasoline, some of which escapes into the crankcase as blow-by. Because of a diesel engine’s efficiency (see above paragraph), it may take longer (than a gasoline engine) to get the crankcase oil hot enough to thoroughly dry it out.

You sure about those injection pressures? IIRC diesel injection pressures are more like 2,000-2,300 PSI.

Diesel engines are more durable than typical gas engines because they must be able to endure much greater compression levels and internal pressure. Combustion in a gasoline engine is achieved by squirting a precise mixture of gas and air into each cylinder, then a spark plug generates a spark to ignite the fuel/air mixture. That small explosion is what forces the piston to move up and down in within the cylinders.

Instead of using a spark for combustion, diesel engines rely on the heat generated by much higher levels of compression to ignite and burn their fuel. They don’t have spark plugs. But the engine also must be capable of handling the high compression levels, which is why diesel engines have historically been very durable and longer-lived that their gasoline counterparts.

But there are some other cold start issues to consider that are fairly unique to diesels. One is that diesel fuel starts to congeal below 40 degrees Fahrenheit and it doesn’t combust well (or at all) in jelly form! As recently as the mid 80s, most diesel engines had “glow plugs” in he fuel tank to heat the diesel fuel on cold days. You would turn the ignition key forward to the ‘Run’ position (but not all the way to ‘Start’) and a light on the dash would come on indicating that the glow plugs were warming the fuel. When that light went off, you could go ahead and turn the key to Start and start the engine.

The vast majority of diesel engines used in cars over the last 20+ years have been turbocharged. Diesels inherently make their power at low engine speeds but they do not rev as quickly or nearly as high as gas engines. A diesel may ‘redline’ at 4000rpm while many gas engines can rev to 6500rpm or beyond. The problem is that diesels are good at getting things moving from a stop, but acceleration is generally much slower than a regular gas engine. In the early 80s, an Oldsmobile Cutlass Supreme with a 350 V8 Diesel took more than 18 seconds to hit 60mph. The same car with a smaller 307 V8 gas engine could do the same in about 12 seconds.

The solution to improving diesel performance to the level of similarly sized gas engines is to add a turbocharger. In the simplest terms, turbochargers are usually made from ceramic and they force a much greater amount of fuel/air mixture into the combustion chamber so the engine generates more power. Ceramic turbochargers operate at extremely high temperatures and they do NOT respond well to operating at sub-freezing temps until they have a few minutes to reach a normal operating temperature. Many new turbocharged vehicles, gas and diesel, have their engine control modules programmed so that the turbocharger will not operate until the engine reaches normal operating temp.

I said all of that to say this- because of their “overbuilt” design and construction and generally lower engine speeds, diesels are less susceptible to cold-start wear than gas engines. BUT they are still susceptible to some degree of cold-start wear and letting them warm up after starting or driving them very gently until the engine temp reaches normal levels will extend the longevity of the engine, gas or diesel!

Yes. I have worked in test cells with production common-rail diesel injection systems operating at 26,000 psi. As the Wikipedia pages indicates, some systems operate at even higher pressures. Emissions rules are what has driven the change in diesel injection pressures over the years. Higher pressures allows the fuel to be driven through smaller holes in the injector at higher speeds, resulting in better atomization and distribution of the fuel in the combustion chamber, which in turn results in lower engine-out emissions.

No problem my experience pre-dates common rail systems.