Why am I looking at temperatures and other thermodynamics?
It seems like I have had a lot of questions around temperatures and why I bother looking at them. The turbo super heats the air and the intercooler cools it back down so it doesn’t matter what temperature the air goes into the intake. Right? Why don’t we find out. I am going to go a little bit into the weeds and dive into some thermodynamics on the intake side of these 6.7L Powerstrokes. I will do my best not to get into what goes on inside the cylinder and exhaust side of it to stay somewhat on topic. I may be repeating some things you all may know or it may be new information. You can decide if you want to read, skim, skip, or flat out ignore this post. Please call me out if I didn’t do a good job explaining something. If I screwed up the science, school me on it. I’m here to learn too and would hate to spread misinformation. All my equations use metric units (Pascals for pressure, Kelvin for temperature, grams for mass, cubic meters for volume, and mols for number of molecules) I have done the math and converted to english units. I will try to attach a spreadsheet with my calculations to check my work or play with if you trust my equations. Just change the values highlighted in green. Let me know if you find something that doesn't work, doesn’t make sense, or is plain wrong.
I’m going to skip around a bit and start with the end of the intake system, the cylinder. Diesel combustion is different from gasoline combustion in a few ways but I want to highlight a major point. Diesel combustion has no real downside to a lean condition as far as fuel burn. In fact it’s almost preferable to have more air (oxygen) than necessary to allow for plenty of air to completely burn with the fuel and give more air to absorb combustion temperatures keeping egts from climbing too high. Adding more air has power and efficiency benefits (more complete burn), particulate emission benefits (more complete burn means less unburnt soot), higher power potential (more air could burn with more fuel making more power), and possibly reliability (lower exhaust temperatures).There are many other balances at play that may cause an OEM to keep air from entering the cylinder. Things such as higher NOx emissions, costs, harder to reach regen temps, etc.
Back to the cylinder. The displacement is going to be the first limitation of air. There is only 6.7L available to be able to cram air into every two revolutions (4 stroke engines hold the air for two revolutions as it works through the different strokes). Imagine for a moment that we have the heads wide open to the air. The most air we can push into the engine in two revolutions without some intake and exhaust scavenging wizardry is 6.7L. At what 70F ambient temperature and 14.7 psi ambient pressure that is only 0.0177 pounds of air with only 23% or so being oxygen. One of the great things about air is that it follows the ideal gas law, Pressure x Volume = number of air molecules x a gas constant x Temperature. This means if we are limited to a volume, we can still increase the mass of air (number of molecules) inside that volume by increasing the pressure of the air or decreasing the temperature of the air.
Let’s start by increasing pressure. There are a couple of ways of doing it but I won’t bother with the positives and negatives of different superchargers and turbochargers. I’m going to stick with the diesel industry standard, turbos. Turbos boost the pressure of the air from ambient (approximately 14.7 psi) to something higher. Different turbos do this to varying degrees of success but I won’t go too much into depth on that. What we need to know is that when the air is compressed it heats up. There is no free lunch in thermodynamics. The equation I have for a temperature increase from turbo is Temperature increase = ((((Pressure out/Pressure in)^0.283) - 1) x Absolute temperature in) / Turbo efficiency. What we can gather from this is the higher the boost the more heat that is made, the higher the temperature in the higher the temperature out, and since I have yet to discover a completely efficient turbo, the turbo inefficiency adds more heat. Like some of the first twelve valve cummins let’s just slap a turbo on and pipe it straight into the heads. At 20 psi boost, 70F ambient temp, and a turbo efficiency of 80% (pulled out of thin air), the temperature out of the turbo is 252.17F. But with the increase in temperature and pressure our mass of air for two revolutions is 0.0311 pounds. Not too shabby. But what else can we do?
Let’s drop the temperature of the air out of the turbo. Intercoolers or charge air coolers (CAC) do just that. They take the warm air and run it past a fluid, either air or liquid coolant, to exchange heat from the warmer air out of the turbo to the cooler fluid of the intercooler. There are other methods to cooling air such as water injection. I’m going to stick with the factory equipment. The air to water style intercooler in the 6.7L Powerstroke helps to limit the amount of pipe routing and pressure loss, using coolant boosts the efficiency of the intercooler, but since the coolant then passes through a radiator the coolant will always be warmer than ambient air. The equation I have for an intercooler is Temperature drop = (Temperature in - Temperature of the fluid) x intercooler efficiency. What we can gather from this is the larger the difference in temperature the larger the drop and since I have yet to discover a completely efficient intercooler, the intercooler inefficiency adds more heat. The inefficiency shows up more the larger the difference in temperature meaning the higher the temperature in the higher the temperature out. There is a possible case where the turbo is adding very little boost and the air to water intercooler actually warms the air slightly. Let’s add the intercooler in between the turbo and the heads. Since the turbo is cramming air into the intercooler it is maintaining the pressure and we don’t see a drop in pressure due to the drop in temperature. At the 252.17F air coming out of the turbo, 75F coolant temperature (an educated guess), and an intercooler efficiency of 80% (pulled out of thin air) the temperature out of the intercooler is 110.44F. With the temperature drop our mass of air for two revolutions is 0.0388 pounds. We have over twice the amount of air we started with. Now for the last piece of the puzzle.
Obviously we want to source air to the turbo and prevent dirt and debris from entering the system. So we add an air filter to keep stray birds out of the turbo. Filters cause a loss of ambient pressure as some work has to be done to draw the air through it. Effectively you are decreasing the ambient air pressure. We’ve also been paying attention and noticed that air density is dependent on temperature. We’ve also noticed that for both the turbo and the intercooler, the higher the temperature in the higher the temperature out. So we try to route to the coldest air available, ambient air outside of the engine bay. We find that we have two competing solutions. One air intake uses a small restrictive filter but is able to fit in a spot where it only collects ambient air. The other intake uses a large much less restrictive filter but can pull some warmer engine bay air. The first intake has an intake temperature of 70F and a pressure drop of 1 psi (pulled out of thin air). The second has an intake temperature of 72F and a pressure drop of .5 psi (pulled out of thin air). Doing the math over for both of these cases the first intake ends up having a mass of air for two revolutions of 0.0298 pounds. The second intake has a mass of air for two revolutions of 0.0303 pounds. I think this clearly shows that 1 psi of pressure change has much more impact than 1 degree F in temperature change. And similar to the data I have and what others have seen with highway driving, it is better to have a free flowing intake even if it means pulling slightly higher intake temperatures.
Let’s take the less restrictive filter and put it in two new scenarios. The first is we go above and beyond to make sure we have nothing but ambient air. The second is we don’t even worry about it. Both are sitting stopped with no air flow under the hood building boost ready to launch for a drag race or with full GCWR trying to make it through the intersection. Because the engine is working hard the underhood temperatures are high. So this first scenario we would see an intake temperature of 70F leading to a mass of air for two revolutions of 0.0305 pounds. This is only 0.66% more air than the scenario before. Hardly looks like it would be worth the effort to block off engine bay heat. For the second scenario we would see temperatures of 110F (a 40 degree rise above 70F doesn’t seem too far fetched from the near 60 degree rise I saw above 40F) leading to a mass of air for two revolutions of 0.0283 pounds. This is less than the more restrictive filter in a situation that may be more important to someone than just cruising down the highway.
There are other methods to add even more air density. Compound turbos to boost the already boosted air. Nitrous (NOS) both increases the concentration of oxygen and has a cooling effect when injected.Water/methanol injection also has a cooling effect. Using ice to decrease the fluid temperature in the intercooler. Along with other tricks.
As you saw I don’t have exact numbers for the system. The intake system is also dynamic and more complex in reality than what I have laid out here. Similar to the filter there are pressure losses through the piping and intercooler. The faster you move air through the piping the higher these pressure losses are. The turbo’s efficiency depends on how much air it’s moving and what pressure it’s compressing to. Engine load can influence the amount of air requested. RPM changed the amount of volume per minute available. Other temperature influences under the hood can raise or lower temperatures moving through the system. These trucks add cooled recirculated exhaust gases back into the airstream at different rates which affects oxygen concentrations and air temperatures. The outside weather can change what ambient pressure, amount of oxygen, and ambient temperature is available. Along with many other factors. This makes it difficult to simulate our exact engines and know exactly how big of an impact certain changes will have.
This is why I am looking at temperatures with the intake. Not because I think dropping intake temperatures 2 degrees will completely change the truck for the better. But because I know that in principle, keeping temperatures as low as I can will keep the truck headed towards an ideal case. If I don’t bother looking at temperatures they can quickly compound and get out of hand, leading me away from that ideal case and possibly worse off than the stock system. I am also attempting to remain grounded in reality, making myself justify time, effort, and cost for any benefits gained. My current data gathered shows there is not enough benefit to gain from the effort of adding shielding to my intake for my purposes. But I still have many other cases to cover and lots more data to collect. I hope this clears some things up.
