Kalter Motor unter Last im Drehzahlkeller = schädlich? | Posts 16+

ulf wrote:

Both are correct - and as long as the air is hotter than the engine block, it will transfer heat to the latter. Although it's only for fractions of a second, but still... the air pump also gets warm continuously when inflating a bicycle tire

.

As a result, after the expansion, the air-fuel mixture has a temperature that is lower than the initial temperature by an amount equal to the heat it released to the cylinder walls as warm air. And in doing so, it cools the engine.
In the balance, it actually comes to zero, unless the air exiting is colder than the air entering. In that case, it would have released heat.
In a bicycle pump, air leaves the pump under pressure. Therefore, heat can be transferred to the pump overall because the expansion that would normally dissipate heat is absent.

Best regards,
Uwe

Uwe wrote:

Therefore, after the expansion, the air package has a temperature that is lower than the initial temperature by an amount equal to the heat it released to the cylinder walls as warm air. And in doing so, it cools the engine. In the balance, it actually balances out to zero, unless the air exiting is colder than the air entering. In that case, it would have released heat. In a bicycle pump, air leaves the pump under pressure. Therefore, heat can be transferred to the pump overall because the expansion is missing.

Hi Uwe,

Yes, of course you're right *slaps forehead*.

I could have also come up with that myself with a little more thought .

Hello Ulf,

I once read a technical article (I think in the MTZ magazine) that investigated how the heat loss in the engine is distributed between the cooling water circuit and the exhaust gases.
The result was that as the engine load increased, the proportion of waste heat that needed to be dissipated through the coolant also increased.
Überträgt man diese Erkenntnis auf Deinen Anwendungsfall, dann wird der Motor im 5. Gang schneller warm als im 4. Gabbreviations
In 4th gear, the catalytic converter reaches its optimal operating temperature faster, and the exhaust system warms up more quickly.
This connection is also "wired" into some automatic vehicles (DC) within the EDC system, causing them to intentionally operate the cold engine at subjectively very high RPMs, solely to warm up the catalytic converter as quickly as possible.

I also observe this "engine warm-up phenomenon" on my usual route. It's only after the first incline, where the engine load increases sharply, that you can really talk about a significant amount of heat entering the cooling system.

Perhaps a few more words about oil pressure and plain bearings. The bearings in the engine are hydrodynamic plain bearings. The load-carrying capacity of the bearings is achieved solely through hydrodynamic effects, and the oil pressure on the supply side only needs to be high enough to compensate for the leakage of the bearing. There should be no problems even in hot idle conditions.
What I mean to say is that I don't attribute much importance to oil pressure. A good example is the lubrication of the rocker arms on the camshaft. These bearing surfaces are only lubricated by splash oil (zero oil pressure!!!), and even here, the hydrodynamic effect ensures that the oil film separates the camshaft lobe and the rocker arm.
When the engine is cold, the oil is very highly viscous and thick, meaning the load-bearing capacity of the lubricating film is many times higher. However, high relative velocities in the lubricating gap can cause the lubricating film to break down when the oil is cold.
Therefore, I think that Ulf's driving style is absolutely fine.
However, in the described case, the difference between 5th and 4th gear is so small that there is no risk to the engine in either situation.
I always try to keep the engine speed below a maximum of 2000 RPM when the engine is cold, although the load can sometimes push it into higher ranges.

Regards,
Alex.

I think it's actually the opposite. The power output is always the same at the same speed. If you drive in 4th gear instead of 5th gear, the overall efficiency of the engine decreases, among other things, due to the lower load and higher frictional losses. This becomes clear when you consider fuel consumption. At the same speed, fuel consumption is higher in 4th gear than in 5th gear. When more fuel is burned, more waste heat is generated, most of which is transferred to the coolant. Therefore, the coolant will heat up faster in 4th gear.

Best regards,
Uwe

@Uwe: While you have lower frictional losses at high load and low speed, there's a counteracting effect: At very low speeds, gas leakage losses become increasingly significant. This is due to the gap between the piston rings and cylinder liners, as well as the valve clearance, through which working gas can escape. Additionally, with conventional fuel injection pumps, the pump pressure at extremely low speeds approaches the opening pressure of the injector, which reduces the fuel injection volume flow (while the injection duration increases). This can negatively affect the fuel atomization, leading to increased soot formation. However, soot is a type of loss that doesn't contribute to heat generation.

Just two thoughts. The extent to which these factors actually become significant at low speeds depends on the design and condition of the engine.

Hi,
Please remember that diesel engines are not throttled. Therefore, at higher engine speeds, more cool air is drawn into the engine than at lower speeds in a higher gear.
Alternatively: The amount of fuel injected per cycle is lower, which consequently results in lower temperatures in the combustion chamber and the exhaust system.
That should compensate for the increased friction heat.