Starting cars--the effect of engine temperature

With winter upon us, I got to wondering about something.

We know that cold weather reduces the effective capacity of car batteries. But when you drive a car for a few miles, then let it sit for 20-30 minutes, the engine is still somewhat warm. Assuming that the weather does not change, is a warm engine easier to start than a cold engine?

Oil viscosity is a key issue. On very cold mornings, the starter motor may lack the capacity tol deliver more than a few grunts, trying to drag the moving engine parts through a tar-like lubricant. Once that warms up, at any time of the year, ,the engine will retain running warmth for several hours.

Back in the days when you had to reset the valve tappets, you had to let the engine cool for 3-4 hours, and reset them cold…

I can tell you that it’s absolutely easier to start a warm diesel than a cold one.

regarding valve adjustment - some cars you do cold (like Honda’s), some cars you do hot (like Toyota’s & Nissan’s), some cars you can even do it while they’re running (old Chevy’s - although it’s less messy & more accurate to do them with the engine off if you really, really pay attention).

you’ve obviously never kick started a 500cc 2 stroke dirt bike.
yes, they should have an easier time starting when they’ve been warmed up. with today’s fuel injected, thin oil having, quick starting vehicles, how much you’d notice this would be negligible.

the older your car, the thicker the oil, the colder it was outside, the more you’d notice it.

In addition to the oil temperature and viscosity that’s already been mentioned, understand that battery temperature influences how much juice it can put out.

So a battery that’s been sitting overnight in a stone-cold car will put out less electricity than that same battery after sitting next to a hot running engine for half an hour. The battery will soak up heat from the warm(er) environment under the hood.

As well, while current is flowing through the battery, that is warming the battery from the inside out. So the act of cranking the car will heat the battery innards a bit. Once the engine starts, the recharging process over the next several minutes will also heat the battery innards.

Both these effects, warmer oil *and *warmer battery help the second cranking effort to go much better than the first.
In older cars there was much less effective fuel atomization in a cold engine. And a weaker spark. All of which meant that in addition to wimpy cranking you were trying to start the fire with less good fuel & heat. In all, stone cold starts were a quadruple whammy.

Modern fuel injection doesn’t much care about the fuel temp or injection plumbing temp; you get great atomization no matter the temp. Modern ignition systems also put out full-power sparks when cold even if the cranking RPM is low. So these issues don’t matter nearly as much as they did with a pre-1980s car.

atomization isn’t really a big part of it; the fuel injectors start spraying fuel while the intake valve is still closed. on a warmed-up engine, the intake valve is hot and causes the fuel to flash-vaporize, and will thoroughly mix with the incoming air as it’s pulled into the cylinder. when cold, it doesn’t evaporate as well so even with port/direct injection engines still need cold-temperature enrichment. it’s just done automatically by the PCM instead of with fiddly chokes.

A few winters ago during frigid weather, a friend stopped in to visit me at work.

When he left, after an hour or so, I walked him to his vehicle. I was surprised to see he’d left his truck running the entire time. He told me it was a diesel, and that’s how you did it.

unless it was an older truck w/o cold start aids all he did was waste fuel.

It’s [del]amazing[/del] entertaining how much guys with diesel pickup trucks want to emulate guys with diesel Freightliners.

Size anxiety?

:smiley: Knowing the truck owner, I’d say yes!

In the 60s, when VW batteries were under a back seat cushion, I and several other people I knew in Montreal popped the battery out and kept it in the house overnight. It made no noticeable difference.

Here in Minnesota, A lot of people used to have ‘tank heaters’ on their vehicles. The cord hung out the front of your car; you plugged this in to a 120V outlet about an hour before you went to start the car. It warmed up the oil in the engine and made it much easier to start.

Not used nearly so much anymore.
But all those outlets in garages & even employee parking spaces give us a head start for recharging electric cars!

Wouldn’t there be an exception to oil like 5W-20 and as such? The 5W means the weight is 5 when cold and increase to 20 weight when warmed.

The 5 is thinner than 20 so it would be easier to start when cold than the straight weight oil like 30 and 40.

Sorry, but this isn’t correct.
The 5W-20 means that the oil has the viscosity of 5 weight oil when cold, and 20 weight oil when hot, but cold 5 weight oil is still thicker than hot 20 weight.

To second Beowulff, the viscosities of the regular and W oils aren’t even measured the same way. SAE J-1306 (couldn’t get a cite to work, sorry) shows the W oils from 0W to 30W measured as temperature to meet a certain viscosity in centipoises, and the non-W oils to be centistokes at a specific temperature. Any viscosity oil gets thicker when cold and thinner when hot. A good multigrade oil thins less (or thickens less) than a straight oil. The degree of thinning is called the Viscosity Index, or VI. A high VI means less viscosity change with temperature. This is often done by starting with the W grade straight oil and adding a VI improver, I think polyisopropylene is common.

For those of us of a certain age, STP was the VI index improver of choice. When your oil consumption got to certain level (about 100mi/qt, the point where you would stop for oil about every gas tank full) you added another can of STP. Unfortunately STP also had a thickening effect at low temperature. Below about -10F you could take the top off the can of STP and turn it over and wait for it to flow.

It depends on just how frigid it was out that night.:eek:

If it was below -30 C, Even the newer diesels have troubles starting from a dead cold.

However, I suspect that the real reason why he/she left it idling is so that the truck will still be warm in the cab when leaving.

The other factor is that a diesel at idle does not consume very much fuel. However, at the -30C or colder temperatures, the newer diesels will automatically idle up to keep the engine warm as the typical low idle speed (700- 800 rpm) does not make enough heat to keep warm and this idling up does consume a LOT more fuel.

I found this out with a 1994 Ford diesel that did NOT idle up automatically - I found a good use for the First aid kit as it was the perfect weight to manually idle it up (Yes it was fuel injected) but a Ford Diesel a few model years later did automatically idle it up (Same basic engine, just with a turbo) so the manufacturers also learned as well. This is also why the manufacturers also started pitting hour gauges in the pickups as well.

The secondary reason for the automatic idling up is running a diesel engine on low idle for hours on end slowly starves the valve train of full lubrication. I found that with the 1994 Ford that the cylinder heads and valves were extremely worn from excessive low idle. (I am referring to low idle in the thousands of hours):smiley:

Is this clear-cut? I’ve had Toyota diesel pickups with weak batteries that started OK when cold but sometimes had trouble re-starting when the engine was warm from running.

(Of course “cold” is relative. I live in the tropics, and ambient temperatures are generally warm or worse.)

It is extremely clear-cut. Even modern fuel-injected gas engines are harder to start in cold temperatures. I find that my car starts turning over noticeably slower at about -20C, and by -25C I’m really wishing I’d plugged in the block heater. I have started it cold down to somewhere between -30 and -35 and it’s never failed to start, but at that point you’re really starting to wonder if it will actually fire, and when it does it kinda clatters until the oil warms up a bit. Cannot possibly be good for the engine.

Actually, I plug in the block heater starting at about -10C, just because the car starts pushing warm air out of the heater a lot faster if I do, and saving a bit of engine wear for the 10 or 20 cents it costs seems like a pretty good deal.

:confused: The temperatures you mention are very far removed from any temperatures relevant to my situation.