Do you like your Diesel warm or cold ?

[Piwoslaw]

Comptiger, so assuming I have a 195°F thermostat, then if I hypermile around without exceeding 80°C (175°F), then my engine will be in small loop mode (no coolant going to radiator) the whole time? Is that OK for the engine?

Would there be any effect in replacing the thermostat with a higher temp unit if my fan kicks in at 95°C (203°F)?

[Arragonis]

Originally Posted by Old Mechanic

I would say if you have plenty of power and a precise method of measuring intake temps, it would be a worthwhile experiment.

Could be an interesting experiement. I have power to spare really (big engine, little car) so I could try this. Does an SG measure intake temps or more importantly log them ?

[comptiger5000]

Your t-stat is probably a 195*. Unless you can bump the fan engagement up to about 210*, I wouldn't change it to a higher temp, as the fan would be on constantly.

Running it cooler with EOC so you're under 195* does keep the cooling loop small. Just let it get up to full temp every few weeks or so for at least 10 minutes of runtime to mix up all the coolant again, and it should be fine.

[ConnClark]

Diesels like cold air.

Lets do a thought experiment. Imagine you have two identical cylinders. One cylinder with 100 grams of air and a cylinder with 110 grams of air. For now lets assume the starting air temperature is the same. Now imagine we add the same amount of heat energy to each cylinder. Some of the energy is going to be transferred through the cylinder wall to the coolant and the rest is going to be transferred to the air. It stands to reason that the air temperature of the cylinder with 110 grams of air will not get as hot because the heat energy is heating more mass. Since the cylinder with 110 grams of air will not get as hot less heat energy will be transferred through the cylinder wall to the coolant and we can extract more of the remaining energy from the air.

Advantage cylinder with more mass in it

Now imagine you have two identical cylinders with 100 grams of air in them. Lets assume that the air temperature in one is 20 degrees C less than the other. Now imagine like before we add the same amount of heat energy to each cylinder. Just like before some of the energy is going to be transferred through the cylinder wall to the coolant and the rest is going to be transferred to the air. It stands to reason that the air temperature of the cylinder that was 20 degrees C lower than the other one won't get as hot because we are heating the same amount of air with the same amount of energy. Since the cylinder that was 20 degrees C lower than the other one won't get as hot less heat energy will be transferred through the cylinder wall to the coolant and we can extract more of the remaining energy from the air.

Advantage cylinder with colder air in it

Now with a cold air intake you get more air in the cylinder and its colder, thus its a win win with a diesel.

This holds true until you cool the air down enough that fuel won't ignite due to temperatures being to low a top dead center. People may argue that since diesels have trouble starting in the winter engines are close to that already but this is not the case. The main reason diesel engines have trouble starting in the winter is the fuel is cold and doesn't atomize well because its more viscous. That is why they thin diesel fuel out in the winter.

There are other reasons why a cold air intake is better as well but these are the two main reasons that are easiest to grasp

[Olympiadis]

Originally Posted by Arragonis

Quick and simple question - WAI for a Diesel - good idea or not ?

Like some other modifications the answer depends on engine loading and your particular combination and driving conditions.

If you always hypermile with very light engine loading, then the warm air will help your fuel efficiency.
If you often load the engine and need more power, then you will want the cooler air.

For many cars this question comes down to the type of driving done. If you're mostly stop-n-go, then it's likely that you'll need the extra power. If you're mostly highway cruising, then you'll likely benefit from the warm air.

Testing each way thoroughly is the best way to determine what your vehicle wants.

Everyone's situation is different, and that's why you'll often get conflicting answers to these types of questions.

[ConnClark]

Originally Posted by white2001s10

If you always hypermile with very light engine loading, then the warm air will help your fuel efficiency.

Got any scientific studies to back this up?

[Olympiadis]

Originally Posted by ConnClark

Got any scientific studies to back this up?

Yes.

[ConnClark]

Originally Posted by white2001s10

Yes.

lets see them

[Daox]

I'd like to see any SAE (or other) testing that shows how IAT effects diesel efficiency.

[Olympiadis]

Originally Posted by ConnClark

lets see them

I'm not going to fall for your fishing any more than you would fall for me asking you to show your scientific data showing that air temperature has no effect fuel efficiency.

The way that you come across in your post is that you simply would not believe when I did show you, and I can understand that.
Hence my statement in the post above:

Testing each way thoroughly is the best way to determine what your vehicle wants.

Now, if you want me to walk you through a revealing test procedure that you can perform to get repeatable results on your own vehicle, thus giving you your own applicable answers, then I will.

I've done my own controlled testing 20 years ago. I have all of my data, but I'm not here to convince anyone, nor am I obligated to answer to anyone. You will always be better off, and more satisfied, if you perform your own testing.

As I indicated above, different vehicle combinations respond differently, and also to different conditions. Differences in engine loading can come down to factors like different drivers, or where the driver lives. That's why I gave a general answer, but then suggest individual testing to confirm. I had hoped that would've been clear.