Relationship between outside temperature and consumption

My consumption statistics show that my TDI (PD) 74 kW engine consumes more fuel when the outside temperature is cold, compared to when it's warm like it is now. My fuel consumption has continuously decreased from a high of 6 liters per unit in the winter to below 5 liters now.
It's unlikely to be related to a cold engine for me, as I mostly drive long distances (>250km).My second suspicion was the fuel temperature. After doing some research on physics, it's probably not that either. Diesel with a higher temperature has a lower density, and my EDC should inject more fuel at a higher fuel temperature.Ultimately, I've come to the conclusion that it's a higher parasitic load (transmission, drivetrain, wheel bearings) that the engine has to overcome.I'm looking for ways to stabilize my fluctuating fuel consumption at a low level.
Can anyone offer advice on this, or confirm, correct, or expand upon my arguments?
Best regards, Peter MH.

Winter diesel has a lower density than summer diesel, which is why more fuel needs to be injected.

Winter diesel has a lower density than summer diesel, therefore more fuel must be injected.

But mp090779 mentioned, '6ltr now at under 5ltr.' That's about 20% .

Is that a significant difference in density? I would have expected a significant increase in fuel consumption for the (albeit few) short trips mentioned by mp090779.

Triggered by.
- the higher viscosity of the cold oils (engine and transmission).
- the electric glow plug heater for the coolant / the heating element for the cabin air.

Furthermore, there is also increased electricity consumption on long journeys due to the greater use of electrical appliances that are typically used during the winter months.
- Low beam headlights
- Heated seats
- Ventilation (possibly a higher setting than in summer).
- Rear window heater
- probably more frequent windshield wiper use (okay, that probably doesn't use even 0.1 liters...)
- etc.

But that would just be my suggestion.

Regards,
Private.

Furthermore, there is also increased energy consumption on long journeys due to the generally more frequent use of electrical appliances during the winter.
- Low beam -> is always on
- Seat heating -> is already too warm for me on the first setting
- Ventilation (possibly a higher setting than in summer).
- Rear window defroster -> very rare, it's more common to scrape the ice
- probably more frequent windshield wiper use (okay, that probably doesn't use even 0.1 liters...)

I don't suspect the electrical appliances as the cause, as even the air conditioning system has hardly any impact on consumption in the summer. I'm more inclined to suspect leaks, especially from the transmission outwards. This would then fully contribute to the pulling power.
My wife, by the way, loves the heated seats (setting 4 -> which is like an egg cooker for me), but she's rarely in the car with me.
The idea of using different diesel fuels for winter and summer is interesting, but it probably has little effect since the fuel consumption doesn't change drastically.

Peter MH.

The diesel fuel is not changed abruptly. There are 5 different blends that can be used, or are required to be used, depending on the season or climate zone.
see DIN EN 590.

Of course, there are also losses in the form of friction (in components like gears and wheels) that could have an impact.
and of course, the winter tires.
The airtight seal could also be factored in...


CU Gremlin.

Hello!

When it's cold, and potentially with higher air pressure (as is often the case in winter with clear weather), you have a significantly higher air density. That means the engine has a better design, and therefore actually produces more power.

Since the introduction of mass airflow sensors in diesel engines, this issue has become even more noticeable than it was in the past. The older diesel engines tended to produce more smoke and soot.

Therefore, the increased consumption and performance can be justified.

The difference in density caused by changes in fuel temperature is compensated for by the control unit, but the type of fuel is not (e.g., winter diesel vs. summer diesel).

Regards,

Knubbi.

Hello!
I can absolutely confirm that! Even the air temperature affects fuel consumption. I always drive the same route, and during the transitional seasons, when it can be cool one day and warm the next, I've clearly observed that fuel consumption depends on the outside temperature. However, this only holds true consistently until the point where the air conditioning kicks in and increases fuel consumption again, because the compressor increases its activity. In my opinion, the best fuel efficiency is achieved with the MIT CLIMAtronic system set to AUTO mode, at a temperature of around +20°C.
There's no doubt that fuel consumption also depends on the outside temperature. In other words, it makes a difference if you drive the same route with a warmed-up engine, once at 10°C and once at 20°C. Not a huge amount, but still quite a bit!

Greetings.

The best fuel consumption is likely achieved when CLIMATRONIC is set to 'OFF,' as the air conditioning compressor is not activated in that mode.

Sure, that makes perfect sense! CLIMAtronic=ECON and the outside temperature as high as possible is optimal . However, since I always drive with CLIMAtronic=AUTO, I was able to observe the turning point at approximately 20°C.

Okay, I don't notice any difference. My TDI weighs 1.8 tons when empty. That doesn't weigh as heavily.

The diesel is also not changed abruptly. There are 5 different mixtures that can be used, or must be used, depending on the season or climate zone.
see DIN EN 590. This is interesting and would fully reflect my experiences.

The winter tires have no noticeable impact on my fuel consumption.

Okay, I understand the physics behind the air temperature (colder air has higher density, leading to more power), and that's why intercoolers are used. However, I'm unclear why, with the same route and driving style, a higher power output should result in higher fuel consumption. During a discussion with Hopa-Tec, they confirmed that if you maintain your usual driving style (normal cruising), there's a possibility to actually save fuel with the increased power. However, this varies from vehicle to vehicle.

'My air conditioning rarely turns on.' I'd rather open the roof.

Okay, there's probably still a theory about that.

With higher density, you get more power, which results in lower fuel consumption because you need to accelerate less to achieve the same speed.

Since humans are not machines and always drive in the same way, it's pointless to discuss this further.

Physics is straightforward, right?

The difference in density caused by changes in fuel temperature is compensated for by the control unit - but not the type of fuel. (Winter and summer diesel).



'It's compensated for by your foot. Lower tire pressure equals less power, so you need to press the gas pedal more to achieve the same performance...'

btw: You can see exactly when I changed the tires on my old car (the 'alt-opa-kiste(tm)') in the usage graph.
I haven't looked into that for the TDI yet, because fuel consumption is hardly a major concern with that model anyway.

CU Gremlin.

MikeF wrote:

I always drive the same route, and during the transitional periods, when it sometimes gets cool one day and warm the next, I have clearly observed that the fuel consumption depends on the outside temperature.

That's also my observation.
A temperature difference of 10 degrees Celsius on my commute to and from work reliably changes the fuel consumption per trip by about 0.2 liters per 100 kilometers.
Even on cold days, no matter how conservatively I drive, I can't achieve the same fuel efficiency as on warmer days, despite consistently having similar traffic conditions and speeds.

I must mention that the individual segment is only about 21 km long, and that, for example, the transmission may not really get warm during that distance.

My observation (according to the MFA 1): Since the new intercooler was installed and the coolant temperature sensor was replaced (although only the side for the instrument cluster was defective), this great PD engine seems to be finally starting to be more fuel-efficient. According to the MFA, the fuel consumption has decreased by 0.3 - 0.5 liters on a single trip. I'm not particularly interested in whether this is related to the rising outside temperatures, but it was an interesting observation to see how much more fuel the turbocharger consumes when it's pumping air into the atmosphere. However, it's also possible that I've somehow learned overnight how to properly distribute the extra 300 kg of weight to make it roll smoothly.

Best regards, WarLord.

WarLord wrote:

but it was an interesting observation, how much more a charger consumes when it pumps air out. It is also possible that I have learned, perhaps overnight, how to properly use that extra 300 kg of weight to make it roll.

The latter is probably less likely (certainly not as suddenly) *gg*.

How big was the hole in the liquid cooling system?


Addition to my previous post:
"Recently, I've been occasionally performing rolling resistance tests on a daily downhill stretch: I shift into neutral at exactly 70 km/h as I pass a specific road sign, and then check the speedometer again at the bottom of the hill."

On two consecutive days, I reached speeds of 84 km/h and 88 km/h. Both times, there was practically no wind + the same trailer weight, and the road was dry.
The only difference is a few degrees Celsius in the outside temperature, and probably also the starting temperature of the transmission when coming out of my garage.
-> The warmer it is, the easier the Polo rolls... and that's quite noticeable. The differences seem bigger to me than they were in my old Golf 3.