Kurzstrecken im Winter...wie am besten fahren?!? | Posts 32+

Wolfgang, syncro16 wrote:

In practice, the cooling effect is likely to be minimal; however, the calculation shows that the majority of the engine braking energy (the difference between E1 and E2) goes back into the engine and not into the exhaust! The energy released during downhill driving can be considerable, especially in a 2.5-ton bus like mine. The braking system would certainly overheat if it had to handle that amount of energy. Hello. Wolfgang

Hello,

Calculate the amount of air, in cubic meters, that is pumped through the engine per minute.

Without pressure, at 3000 RPM, approximately 2800 liters per minute (for a 1.9-liter machine, 100% fill).
With a specific heat capacity of 1.29 J/g for air and a density of 1.2 kg/m³, the power required is approximately 72 W per degree Celsius of temperature increase.
Assuming a temperature difference of +60 degrees between the air outlet and inlet, 4.3 kW of thermal energy is released into the exhaust.
What is the approximate size of a large diesel auxiliary heater?

Best regards,
Christian.

Hello Chris,

How did you arrive at 60°C? The engine is cold, at least initially.
Wenn ich ein 10% Gefälle mit 36km/h hinabfahre, dann entspricht das 1m/s Fallgeschwindigkeit und es werden 25kN x 1m/s = 25kW Leistung aus der potentiellen Energie frei. Where are they going?
The engine braking process can cause the engine to warm up significantly, potentially reaching its operating temperature. While some of the heat will eventually be released through the exhaust, a certain level of heat will remain, and this equilibrium point can be quite high depending on the specific circumstances.

Hello.
Wolfgang.

Hello chris,

How did you arrive at 60°C? The engine is cold, at least initially.
Wenn ich ein 10% Gefälle mit 36km/h hinabfahre, dann entspricht das 1m/s Fallgeschwindigkeit und es werden 25kN x 1m/s = 25kW Leistung aus der potentiellen Energie frei. Where are they going?
The engine braking process can cause the engine to warm up significantly, potentially reaching its operating temperature. While some of the heat will eventually be released through the exhaust, a certain level of heat will remain, and this equilibrium point can be quite high depending on the specific circumstances.

Hello.
Wolfgang

Hello,
I don't believe the motor can reach its operating temperature by driving downhill at 36 km/h!
The cold air drawn in is heated during compression, and some of that heat is certainly transferred to the engine block. However, the temperature is significantly higher during actual engine operation and remains at that higher level for a much longer time!
During the scavenging process, the air cools down as it expands, and will inevitably absorb heat from the engine block again at the end.
Furthermore, the fact that air has entered the crankcase and is not released during decompression causes the air to become even colder than it was initially.
The frictional losses will likely dissipate into the engine oil temperature and be 'lost' in the oil pan.
Let's initially consider the scenario with a completely cold engine.
Best regards,
Günther.

Hello Günther,

So, I don't believe that the engine can reach its operating temperature by driving downhill at 36 km/h.

That's based on my practical experience. It's important to have a proper slope and a 'reasonable' vehicle weight. My 2.5 tons combined with the small AFN engine is certainly helpful in that regard. Higher speeds are more likely to be counterproductive, as too much energy would then be wasted on rolling resistance and air resistance.

Furthermore, a negative effect is that some of the air has entered the crankcase and is absent during the expansion stroke, causing the air to become even colder than it was initially.

I believe that these frictional losses likely contribute to warming, as they definitely slow down the crankshaft, and this lost energy remains in the engine as heat. Furthermore, as you correctly pointed out, the exhaust gases tend to cool down.

Hello.
Wolfgang.

@Wolfgang:

I have corrected my comment accordingly. Of course, my statement referred to the warm engine, which cools down during coasting.
Your observations about the cold engine warming up on inclines are certainly correct, as there is a temperature difference between the combustion chamber and the engine block/cylinder head in this situation (see: my previous post in this thread).
However, I find it a bit hard to believe that you reach the operating temperature (minimum 80°C) solely through idling. I think it's more likely that a steady state is established somewhere around 50-70°C. Have you ever read the temperature sensor using VAG-COM, or does your instrument cluster not have a plateau function?

@Ulf:
The mechanical efficiency of reciprocating piston engines typically ranges between 80% and 85%, which means that a significant amount of heat is released due to friction.
However, this efficiency rating likely (who knows better?) refers to the operating point at the rated speed and rated power.
At low partial load, the mechanical efficiency is certainly lower because the friction is only partially load-dependent. The friction in the main bearings and connecting rod bearings is practically purely viscous friction, meaning it is proportional to the speed and proportional to the oil temperature. The bearing load also plays a significant role here, as the lubricant film thickness varies depending on the load.
The friction between the piston rings is practically independent of the load during the intake and exhaust strokes. During the compression stroke, the current boost pressure plays a significant role, while during the power stroke, the maximum ignition pressure and the pressure profile during expansion are important factors.
The friction in the valve drive mechanism is practically independent of the load and is largely determined by the viscosity of the oil.
You can see that it is not possible to make a precise statement without detailed knowledge and measurements.

Here's a rough estimate: Let's assume the following operating point for a medium load: n = 2000 rpm, M = 120 Nm, P = 25 kW.

Assuming a mechanical efficiency of 80%, 6.25 kW of frictional power would be generated within the engine.
Assuming a partial load efficiency of 32%, the combustion process would supply approximately 78 kW. Of this, an effective 25 kW + 6.25 kW = 31.25 kW would be transferred to the kilowatt meter.
The remaining approximately 47 kW are lost either through the exhaust or via heat dissipation to the cylinder head/engine block.
When comparing the performance, it quickly becomes apparent that the warm-up process is faster when using a lower speed and slightly higher load, compared to the reverse.
The increase in heating power achieved by increasing the engine speed is minimal. While increasing the energy supply through a lower efficiency has a more significant impact, the distribution of heat losses between the exhaust gases and the engine block/cylinder head has the greatest influence.
My assumption now is that at higher loads, there is a significant shift in the proportion in favor of the block, which results in faster heating.

In Wolfgang's case, only pure friction within the unloaded engine is present. At 3000 RPM and with a cold engine (high oil viscosity), this can quickly generate 5-6 kW of heat, which, during a long descent, slowly but continuously heats up the engine, at least to the point where the heat pump effect in the combustion chamber creates a balance between the heat generated by friction and the heat dissipated by the exhaust gases.

Regards,
Alex.

donalexo wrote:

The mechanical efficiency of reciprocating piston engines is somewhere between 80% and 85%.

Hi Alex,

Thank you, a nice and, in my opinion, plausible-sounding contribution .

As far as I recall, this is the first time someone has written so thoroughly about heat balance calculations in engines.

What is the actual heating output of the heating system?

Normally, I turn off the heating until the coolant temperature reaches 90°C (supposedly). It's much faster to do this without the heating on. When going downhill, the temperature only drops if the heating is on. However, without the heating on, the temperature doesn't increase either.

What is the actual heating power of the heating system?

'Recently, when I drove down the Wasserkuppe mountain (-4°C, snow), the auxiliary heater with its 5kW output was unable to maintain the water temperature (the climate control was set to 25°C).'

I would estimate that 5-10 kW is definitely achievable, considering the temperature differences the heating system has to manage.

However, I would also be interested in more specific details.

CU Gremlin.

is now 1 km flat, 3 km downhill, and 2 km in the city.
Now, I'm constantly thinking about how to best drive the short distance so that the engine and the oil are not too negatively affected by the short trips.

How about a bicycle? I mean, 6km isn't that far, unless you have any physical limitations. Besides, you're guaranteed to find shortcuts and alternative routes on a bike that aren't accessible by car. And your lungs will really appreciate the fresh air, your circulation will improve, and you won't gain weight...

Hmm, it's a thought, but I'm wearing a suit because I work at a bank .

donalexo wrote:

Here is just a rough estimate: Let's assume the following operating point for a medium load: n = 2000 1/min, M = 120 Nm, P = 25 kW. Assuming a mechanical efficiency of 80%, 6.25 kW of frictional power would be generated within the engine. Assuming a partial load efficiency of 32%, the combustion would supply approximately 78 kW. Of this, an effective 25 kW + 6.25 kW = 31.25 kW would be transferred to the kilowatt meter. Regards, Alex

Hello,

With a heating power of 78 kW, you would consume approximately 7.4 liters of diesel per hour (heating value of 10.57 kWh/liter). This is correct for highway speeds. For mixed city/country driving, I would estimate about half that amount, as the average speed is likely to be around 50 km/h.

Best regards,
Christian.

@chris11:

With your comment, you are certainly right, because 25 kW (average partial load) is not needed to cruise at 50 or 80 km/h on a flat surface. 8-10 kW of useful power is quite sufficient for that. That is then also more of an operating point with a low partial load .

When operating at a low load, it is even more important to choose the correct operating point, because the small amount of heat that is lost should ideally be available for heating.

Regards,
Alex.

Hello everyone,

During the days when I was offline, a very extensive discussion seems to have taken place, with plenty of theoretical and practical reports.
Great for our forum!

After a lot of reading, I also realized that my 'non-expert ' statements weren't completely wrong. It's a shame that, in some cases, I had to explain the details of my assumptions with a harsh tone. It was certainly due to my non-expert description (I'm more of a practitioner than a theorist) that I was sometimes misunderstood.
As I said, it's a shame for me, but it's very good and helpful for the forum.

I find the contributions from ulf, danalexo and wolfgang, syncro 16 to be logical and understandable. The theory is supported, among other things, by practical experience.
Here's a link that also helped me understand thermodynamic processes in internal combustion engines better. Perhaps it will be of interest to others as well.
http://web.uni-frankfurt.de/fb13/iap/lehre/V_Energie_5.pdf

Best regards, Georg.

Then I'd like to add my two cents here...

The lower the engine speed, the more time is available during the compression stroke for the compressed air to release its heat through the cylinder wall. This is so significant that the maximum pressure occurs as early as 1° before Top Dead Center (TDC). When fuel is injected, the slow expansion at low engine speeds causes a larger portion of the combustion heat to be lost through the cylinder wall. Because higher torque is required at low engine speeds to produce the same power, this necessitates a larger fuel injection quantity, which increases the temperatures inside the cylinder, further amplifying the heat loss. Furthermore, the amount of diesel injected per volume of intake air is higher at low speeds, excluding the effect of turbocharging. Therefore, I would recommend operating the engine at lower speeds to warm it up.

When rolling downhill, one should not neglect the flow losses. Because the air has to squeeze through the channels and valve clearances, significant friction is generated, which is on the order of magnitude of mechanical friction. This causes both the air and the engine to heat up, so that the air ultimately comes out warmer.