Kurzstrecken im Winter...wie am besten fahren?!? | Posts 16+

Hi,

I've often asked myself the same question... My Audi S4 also has to deal with daily commutes of 2x10 km in the city, and occasionally gets to drive longer distances on the weekends. This morning, the oil temperature rose to about 75 degrees Celsius, with a starting temperature of -2 degrees Celsius and extremely gentle acceleration.

Otherwise, I do as Ulf does – I use minimal RPMs (1200 in 5th gear, because I'm not allowed to go any faster) and just before the university, at a crossroads, I shift into 2nd or 3rd gear to prevent the VTG (variable torque generator) from seizing up.

The coolant doesn't reach temperatures above 90 degrees Celsius during these trips.

Best regards,

Jan.

gwg wrote:

The engine will certainly absorb some heat from the compressed, hot air during the compression stroke; however, during the expansion stroke, it will also lose heat to the now cooling air. Let's not consider minor nonlinearities for now. Otherwise, the engine would operate as a "heat pump" in gliding mode.

Yep, that's also a way to express it more scientifically .

Yes, but to compensate, the diesel engine is also built more robustly (with larger bearing diameters, etc. - I hope so...).

And? For that, the diesel engine has more mass that needs to be heated. Diesel engines often still have cast iron blocks, while gasoline engines are almost exclusively made of aluminum. Cast iron conducts heat so poorly that it can quickly lead to stress on components. (remember the 1.6d in the Golf2 -> endless block cracks).

The diesel engine has the significant advantage of cold starting because it does not experience a rich combustion and it has less condensation of fuel on the cylinder walls. However, because it injects so little fuel, it also generates less heat.

by the way

We've discussed this cold start issue countless times. Does it really have to start all over again every winter? -> Use the search function!

btw2/

For 2 times 6km per day, a diesel engine is not worth it!

ulf wrote:

If it ever stops working, I know that the amount of diesel (in the oil) being pushed into the intake passages by the fuel pump + KGE is sufficient for the LL. . . .

With the help of the shut-off valve, you should be able to cut off his air supply...

joegolf wrote:

However, thanks to the shut-off valve, you should be able to cut off his air supply...

If it really seals tightly enough to allow for parking...

I agree with Julian; this discussion has already happened many times.

However, even with all your theories, one thing shouldn't be overlooked: When the engine is idling, no fuel injection occurs, that's clear. You've already discussed the temperature change caused by the compression process. The potential energy has also been mentioned.
However, when you drive down the hill in 6th gear (or 5th... it doesn't really matter), the car is "slightly" slowed down, which doesn't happen when it's decoupled. So, the engine absorbs some of this energy, which we can call "deceleration energy." As described, the sum of all energies is always zero, so this energy must be dissipated. This happens through friction. Since friction (and also the compression work) ultimately produces heat, the engine should technically become slightly warmer even in coasting mode. Okay, this is a very minor factor, especially since the hill would have to be infinitely long to prove it. Unfortunately, the significant amount of wind at these speeds also contributes to the problem.

"For such short distances, I wouldn't primarily rely on diesel in the winter, because the electricity needed for the starting process and the subsequent journey likely can't be replenished by the alternator at a maximum of 1500 rpm over such a short distance, and the battery would quickly run out."

It's best to get a cheap winter car .
Short trips like that are murder on a diesel engine.


I can refute that...

The evidence is located in the courtyard below.

- Mercedes MB-100
It's started every morning and driven 3km to the workplace, and then driven back home in the evening. The engine is in excellent condition.

- BMW 525td (very old model); a car typically used by housewives. Used for trips to the bakery, kindergarten, and back. And believe me, the car gets put through its paces...


calvin:

It's not a problem. Take the A3 towards Würzburg, turn around at Marktheidenfeld, and go back up into the Spessart. Do this once a week, and the loader will stay nice and clean .
Otherwise, just shorten the maintenance intervals.

'My colleague was apparently speeding down the mountain in his ASZ, flooring it, 'to warm up the damn car.' If I had bet against it, I would have been a poor gremlin. The ASZ does need a little more oil (1 liter every 16,000 km), but otherwise it's in great condition... currently at 56,000 km.'


CU Gremlin.

Hello everyone!

Perhaps some further thoughts on the thermodynamics of the diesel engine in idling mode .
As long as the engine is very cold, heat is transferred to the engine block/cylinder head during the compression stroke (keywords: temperature gradient, entropy).
After the expansion stroke (work stroke without injection), the cycle then ends under vacuum. This is not a major issue because the piston immediately pushes back again.
When the engine is warm, the temperature gradient reverses. Certainly, the temperature at the compression end is still significantly higher than the block temperature, but I suspect that for a large portion of the stroke, the temperature gradient is the opposite (which still needs to be proven through calculation!).
Anyway, I believe that the sliding operation, when the warm (EDIT 18.12.2004) engine is running, cools it down (heat pump).

Here's something else on the topic: The distribution of heat losses through coolant and exhaust gases is operating point-dependent. In principle: The higher the load, the more thermal energy goes into the coolant, and the less comes out through the exhaust .
Therefore, it is doubly beneficial to avoid operating a cold engine at high RPMs with low load, and instead to operate it at low RPMs with moderate load.
Connecting rod bearings and crankshaft bearings can handle high loads at low engine speeds because the oil viscosity is still very high at those speeds.

Regards,
Alex.

donalexo wrote:

Anyway, I believe that running the engine in "idle" mode with a cold engine cools it down (heat pump). Best regards. Alex

Sure.

Anyone can observe this if they let a diesel engine coast down a long hill (perhaps with a trailer attached). The engine will noticeably cool down.
High engine speeds amplify the effect because the increased airflow in a diesel engine carries away more heat from the air.

Lower engine speeds during partial load operation cause a diesel engine to warm up faster than higher speeds, even though it might seem otherwise.

Best regards,
Christian.

Hi,

Ideally, a standing heater would be best, or at least a garage. That would solve many problems (like ice scraping, a cold engine, etc.).

Unfortunately, mine doesn't have a heater.

Best regards, Christian.

@Polo2004
The 2.2 bar in your signature is probably not quite right. Do you mean 1.2 bar, perhaps?

Edit ulf:
If he is referring to absolute pressure, then 2.2 could very well be correct .

donalexo wrote:

The distribution of heat losses between cooling water and exhaust gases is dependent on the operating point. In principle: The higher the load, the more heat energy goes into the cooling water, and the less comes out through the exhaust. Therefore, it is doubly beneficial to avoid operating a cold engine at high RPMs with low load, and instead to operate it at low RPMs with moderate load. Connecting rod bearings and crankshaft bearings can handle high loads at low RPM anyway, because the oil viscosity is still very high.

Hi Alex,

another theoretical question:
With each engine revolution, friction heat is generated, which initially (mainly) goes into the engine block and pistons.
-> The higher the speed, the greater the "frictional heating power," which means the opposite trend compared to the distribution of combustion heat.

Do you have any information on how this calculation (in general) works out?
Is the amount of frictional power loss negligible compared to the combustion heat (at partial load)?
-> For example, when driving at 80 km/h on a flat road, is it better to warm up the engine faster by driving in the highest gear, or by downshifting?

Hello,

chris11 wrote the following:

donalexo wrote the following::

Anyway, I believe that running the engine in 'idle' mode with a cold engine cools it down (heat pump).
Best regards.
Alex.


Sure.

Anyone can observe this if they let a diesel engine coast down a long hill (perhaps with a trailer attached). The engine will noticeably cool down.
High rotational speeds amplify the effect, because the increased airflow in a diesel engine transports more heat away from the air.

I don't see it that way. When driving downhill, the normal engine cooling is initially effective, which causes the temperature to drop. This stops when the thermostat is closed. In cars with a front-mounted engine, the airflow through the engine compartment can still provide additional cooling, especially when driving at high speed and low RPM, so that all the braking force comes from air resistance and the engine braking effect is practically zero. Of course, the engine can only warm up (or stay warm) if it is supplied with energy.

I often drive my (rear-engine) bus in the mountains. It frequently happens that you start with a cold engine at the top of the mountain and then have to drive 10km steeply downhill. When driving at a low speed (30-40 km/h) and higher RPMs (around 3000 RPM), thus converting as much of the potential energy in the engine as possible, the engine warms up considerably. I haven't analyzed this scientifically yet , but after about 500 to 1000 meters of altitude difference, the engine is warm.

Schauen wir uns mal den Verdichtungsprozess im Zylinder an: Nehmen wir an der Motor hat am Anfang noch die gleiche Temperatur wie die angesaugte Luft, dann schliessen die Ventile, die Luft wird verdichtet, die Kurbelwelle muß mechanische Energie E1 aufwenden, die Temperatur steigt; dann expandiert sie Luft wieder, es wird mechanische Energie E2 an die Kurbelwelle abgegeben und am Ende hätte die Luft genau die gleiche Temperatur wie am Anfang und es wäre E1=E2, wenn es keinen Wärmeaustausch mit der Zylinderwand gegeben hätte. Weil aber während der gesamten Verdichtung die Lufttemperatur über der Zylinderwandtemperatur liegt, gibt die Luft sicher Energie ab, so daß am Ende tatsächlich kältere Luft zum Auspuff rauskommt und E1>E2 ist. Ich habe mit dieser Vorstellung keine Probleme. Da noch etwas Luft an den Kolbenringen vorbei gepresst wird, verstärkt sich der Effekt noch. In der Praxis dürfte die Abkühlung natürlich minimal sein; die Überlegung zeigt jedoch, daß der überwiegende Teil der Motorbremsenergie (Differenz zwischen E1 und E2) in den Motor und nicht in den Auspuff geht! Die bei Bergabfahrt freiwerdende Energie kann beträchtlich sein, zumal bei einem 2,5t Bus wie bei mir. DiThe braking system would definitely overheat if it had to handle that kind of load.

Hello.
Wolfgang.

Hello,
I would say that a cold engine gets somewhat 'warmed up' when driving downhill.
Similarly, a warm engine will eventually be 'cooled down' by the 'pumped' air when driving downhill, but these 'effects' are quite 'small'!
Best regards,
Günther.

@gwg

I think you're right... weren't we just having this same discussion in another thread? Or am I mistaken?

@Polo2004
The 2.2 bar in your signature is probably not quite right. Do you mean 1.2 bar, perhaps?



Hi,

I'm referring to the absolute pressure that exists in each section.

Best regards, Christian.