1 hour, 1Z idle speed -> cold !!!

Hi.

I just helped a friend deal with the aftermath of an accident (fortunately, no one was hurt).

But to make sure his son didn't freeze, I kept my car running.

The 1Z ran perfectly for over an hour while stationary (with an outside temperature of around 0 degrees Celsius).

As everything was finally clarified and we wanted to go home, I thought that the engine was definitely overheating . The temperature gauge showed "COLD" - literally ice cold - but it was just barely above the white lines on the G3 (so still well below the 70-degree mark).

Could that be possible? Under normal operating conditions (so far, 2,150 km of highway driving logged), it doesn't get hotter than 81 degrees Celsius (according to VAGCOM).

There was a thread here before about a Passat where the 1.9 TDI engine consumed a lot of fuel because it wasn't reaching its operating temperature. Currently, mine consumes 6 liters per 100 kilometers – and that includes a minimum of 100 kilometers of highway driving (at 180 km/h) with each fill-up, as well as a lot of city driving.

It sounds to me like a thermostat replacement is needed – or does a TDI engine never warm up properly when idling?

m;

Hi Martin,

I can easily "cool down" my engine's air-fuel mixture in idle mode during the winter using the heater blower.
That means the heating system can extract more heat from the engine than the engine releases into the coolant.

As long as the upper hose leading to the radiator isn't completely warm all the way through, I think this is perfectly normal.

Hi Martin,

The same can be observed at ASV. When I'm standing at a traffic light in the winter, and the engine is idling with the heater running, the oil temperature on the digital display drops by about 0.5 degrees in a relatively short time - with longer waiting times, that amount is likely to be significantly higher.

Best regards,

Jan.

I agree with Ulf! The efficiency of TDI engines is very high, which is why the heat output is relatively low. I can see this myself with my 96kW TDI. The heater is on full blast, the fan is on level 2 or 3, the engine is idling, and you can watch the temperature gauge drop. You should be concerned if the temperature doesn't go much above 70 degrees Celsius during normal driving. However, if it's around 80 degrees Celsius according to VAG-COM, I wouldn't worry. There are also different thermostats (83 and 87 degrees), and within the tolerances, it's possible that an 83-degree thermostat could read 80 degrees. Also, the temperature sensor is located far away from the thermostat, which adds to the variability.This will cause deviations in the display!

Hello,

"In my Passat, you can already notice it inside the car when driving downhill for an extended period – the heating needs to be adjusted. The older, less efficient diesel engines also didn't get warm quickly when idling." That's normal when the outside temperature is very low.

Does anyone know approximately how much fuel a TDi engine consumes per hour when idling ?

@Haehnlein:

Approximately 0.4 to 0.7 liters per hour.

Hi Julian,

Okay, good to know. If you're driving at 120 km/h, do you need approximately 10 times more fuel per hour? We had a topic here before, something like "Warm-up in idle" , meaning that the engine, when stationary, takes about 100-150 minutes to warm up instead of the usual 15 minutes (given the current outside temperatures and a greatly simplified calculation).

Hi Christian!

Your perspective is a bit too "girly" or "girlish." Sorry!

I will try to provide a more in-depth analysis here to estimate the heat balance of the TDIs.
Assuming an idle fuel consumption of 0.5 liters per hour, the following energy balance results:

Heating value of diesel: approximately 44,400,000 J/kg.
Density of diesel fuel: approximately 0.85 kg/l.

Energy input per hour: 44,400,000 J/kg * 0.85 kg/l * 0.5 l/h = 18,870,000 J/h
corresponds to an input power (chemical/thermal): P_input = 18,870,000 / 3,600 s = 5.24 kW.

If this power output were available solely (e.g., via STH) for heating the cabin, then no TDI drivers or passengers should experience coldness during the winter.
However, a certain portion of this power is converted into mechanical power to overcome the frictional losses of the engine and to drive the auxiliary components. However, it should be possible to recover these frictional losses and convert them back into "useful heat."

Now, 4 effects are responsible for the fact that this immense heating power is not available for heating the cabin.

1. Heat radiation of the unit.
2. Heat dissipation from the unit through heat conduction and transfer to the environment.
3. Heat loss through exhaust gases.
4. Heat capacity of the engine.

Performing a reasonable estimate for points 1 and 2 is not trivial, as it requires knowledge of a large number of parameters.
An estimate for the heat losses through the exhaust gases could be calculated as follows:

Idle speed: n = 900 RPM.
special Density of air: ρ = 1.25 kg/m³
special Specific heat capacity of air: 1.005 kJ/(kg*K)
Estimated exhaust gas temperature at idle: T = 150°C.
Intake air temperature: 0°C.

Air mass flow rate: m' = 900/60 * 0.5 * 1.9 / 1000 * 1.25 = 0.0178 kg/s
Required heat power for heating the intake air: Pheiz = 0.0178 * 1.005 * 150 = 2.68 kW

This indicates that approximately 50% of the heat supplied during idle operation is lost through the exhaust .
Assuming that, during a cold start, the engine block has the same temperature as the surrounding environment (in the example provided, 0°C), approximately 2.5 kW of heating power would initially be available to heat up the block.
As soon as the engine's temperature rises slightly above the ambient temperature, heat is dissipated through thermal radiation and conduction, and the amount of heat dissipated increases as the engine block's temperature rises.

Assuming a mass of approximately 100 kg for the engine, including coolant, oil, and attachments, a simple estimate of the warm-up time can be made based on the thermal capacity of the engine.

It is relatively difficult to determine a specific heat capacity for an engine because the mass is distributed across various materials such as steel, cast iron, aluminum, water, and oil. In the following, I will assume an average heat capacity of approximately 0.6 kJ/kg.
This results in a heat capacity of C = 100 kg * 0.6 kJ/(kg*K) = 60 kJ/K for the engine.

With ideal thermal insulation of the unit against heat conduction and heat radiation, and without heat removal via the heating heat exchanger, the ideal warm-up time in idle mode would be:

t heating = (60 kJ/K * 80 K) / 2.5 kW = 1920 s = 32 min

Now, however, the real-world conditions come into play. As soon as a noticeable temperature difference exists between the object and the surrounding air, a significant amount of heat is released to the environment.
So, letting the cold engine run for an hour in idle might not bring it up to operating temperature.

During the warm-up phase of a moving vehicle, the engine block is additionally cooled by the cold ambient air flowing over it due to the vehicle's speed. This significantly improves heat transfer, which helps to "cool down" the engine.

Regards,
Alex.

Hi Christian!


Assuming an idle fuel consumption of 0.5 liters per hour, the following energy balance results:


Regards,
Alex


And that's where the major problem lies.

In the first few minutes, the consumption will be x times higher.
The colder the engine, the higher the fuel consumption.

Therefore, the heat output will be significantly higher.

Your assumptions are generally quite accurate; it's well-known from the design of CHP (Combined Heat and Power) systems, for example, that heat losses are typically split 50/50 between the cooling water and the exhaust gases.

@leolutos:

Quote:

In the first few minutes, the consumption will be x times higher. The colder the engine, the higher the fuel consumption.

Of course, I completely agree with you. However, the idle consumption will stabilize quite quickly, as you already mentioned (within a few minutes), so that the percentage error caused by this assumption is negligible compared to the required heating time.

Regards,
Alex.

donalexo wrote:

Your perspective is a little too naive. Sorry!

Hi Alex,

I also added that it's extremely simplified (I know that was overly simplistic). But that's why we have the nice professional experts in the forum who can thoroughly explain everything to someone like me, who just does a rough, basic job.

Well, Haehnlein, don't belittle yourself by calling yourself a 'pavement sparrow'... even our Don-Alex-o has forgotten a few things in his calculation, and if you look closely, it's also (just) a very simplistic calculation.
But that goes too far, but as an approach, it wasn't bad!

To make matters worse, the interior has significant heat loss, and the heating capacity, while seemingly high at 2 kW, is not particularly generous. The air is constantly being replaced by cold fresh air, and all the wall surfaces initially have the outside temperature, which quickly dissipates heat.

Even when driving at a slow and steady speed (approximately 70 km/h) over a longer distance, I can easily lower the engine temperature using the interior heating system. That's why I already replaced my thermostat - unfortunately, without any change.

Best regards,
Uwe