Engine insulation for hybrids to improve fuel economy

[Talos Woten]

Originally Posted by Ecky

Just came across this thread, and unfortunately I'm coming into summer, so it isn't a whole lot of immediate use, but I do have a few thoughts to add:

It might be worth exposing an airflow path around the coil packs. These are probably pretty heat resistant, but being excessively warm might reduce their lifespan.

The results speak for themselves, but I have some skepticism about the mechanism by which it's believed to work. An insulated ceiling does virtually nothing if the windows are open, so to speak, and I wonder if the added insulation between engine and hood isn't instead functioning mostly to fill space that air would otherwise be flowing through, rather than actually insulating a heat loss path through the hood.

Hey Ecky!

I first installed the insulation during the summer and have mostly used it in warmer weather. It basically shortened the initial warmup of the vehicle by 45s+ish, and helped it retain heat. So it improved fuel economy by a small amount, especially on short frequent trips. I actually haven't had an opportunity yet to test it in the cold, where it seems like it would perform best, so I'm very interested in hayden55 and Phase's results.

Your skepticism seems unwarranted. By your same logic, insulating an attic in a house would have virtually no benefit because of heat loss through the sides. But if you've ever insulated an attic, the difference is night and day throughout the whole house. Furthermore, even if we accepted your premise of an infinite side heat sink, there is a header gap between the ceiling and the top of the windows. The temperature there would definitely change, at the very least into a gradient.

However, if we restrict ourselves to just observable facts without hypothesis bias, I removed my insulation first because I was intending to put a bonnet vent in. That caused a) my warmup time to increase by 90s+, b) the hood to become so hot I couldn't touch it after driving, c) my fuel economy to go down, and d) my engine temp to waver constantly between 175F and 185F (operating range is 185-190). After installing the insulation, the warmup time decreased, I can touch the hood fine, my mpg went up, and the engine temps were good again. (Right now they are wonky because I've messed with the radiator inlet. That's a separate issue that occurred months after.)

Anyhoo, you may draw your own conclusions from that data. The one that I've drawn is that the hot hood with air flowing over it acts like a secondary radiator, with a significant heat flux through it. Cheers.

[Phase]

since i still havent attached the insulation to the hood yet and have just been laying it down over the engine bay for right now, i can tell you that the underside of the foil is definitely warm and hot, while the top is ambient temp. so its definitely blocking and relfecting some heat underneath from rising, which should translate to holding heat in so far. that would at least mean keeping the engine warmer when stopping to do errands and ev driving in city traffic. havent been able to do an tests on time on how long it takes to fully warm up though...

[Ecky]

Originally Posted by Talos Woten

Your skepticism seems unwarranted. By your same logic, insulating an attic in a house would have virtually no benefit because of heat loss through the sides. But if you've ever insulated an attic, the difference is night and day throughout the whole house. Furthermore, even if we accepted your premise of an infinite side heat sink, there is a header gap between the ceiling and the top of the windows. The temperature there would definitely change, at the very least into a gradient.

What got me thinking along these lines was the experience of living in a very leaky house in winter. I heavily insulated the attic, and under the floor, and it would still lose all of its heat in minutes rather than hours, especially if the wind was blowing, because all of the seams around the windows and doors had high levels of convection cooling. Until I went around and caulked all of the seams, that insulation was next to useless.

Taken as a logical extreme, you can place a double paned window on the ground near a camp fire, and it'll keep you warmer, but only because it's a wind break. The replacement rate for all the air in the engine bay is just fractions of a second if the vehicle is moving even at walking pace.

Originally Posted by Talos Woten

However, if we restrict ourselves to just observable facts without hypothesis bias, I removed my insulation first because I was intending to put a bonnet vent in. That caused a) my warmup time to increase by 90s+, b) the hood to become so hot I couldn't touch it after driving, c) my fuel economy to go down, and d) my engine temp to waver constantly between 175F and 185F (operating range is 185-190). After installing the insulation, the warmup time decreased, I can touch the hood fine, my mpg went up, and the engine temps were good again. (Right now they are wonky because I've messed with the radiator inlet. That's a separate issue that occurred months after.)

I agree 100%. Whatever the reason, it works.

Originally Posted by Talos Woten

Anyhoo, you may draw your own conclusions from that data. The one that I've drawn is that the hot hood with air flowing over it acts like a secondary radiator, with a significant heat flux through it. Cheers.

On my car, even after driving for hours, the hood is not discernably warmer than ambient. I'd wager that insulating my hood would make little difference in terms of heat loss through it until I close up the engine bay and keep fresh air out adequately for the hood to get warm. It stands to reason that you engine bay already has its airflow limited enough.

[Phase]

i have 100 percent grill block so i guess i can still benefit from hood insulation

[Talos Woten]

Originally Posted by Ecky

What got me thinking along these lines was the experience of living in a very leaky house in winter. I heavily insulated the attic, and under the floor, and it would still lose all of its heat in minutes rather than hours, especially if the wind was blowing, because all of the seams around the windows and doors had high levels of convection cooling. Until I went around and caulked all of the seams, that insulation was next to useless.

On my car, even after driving for hours, the hood is not discernably warmer than ambient. I'd wager that insulating my hood would make little difference in terms of heat loss through it until I close up the engine bay and keep fresh air out adequately for the hood to get warm. It stands to reason that you engine bay already has its airflow limited enough.

Ahhhh! This makes much more sense, and is pretty reasonable to boot. Okay, let's go back to the data. I've attached a couple of choice Torque Pro logs. The first is the coldest temperature I could find, which shows a 12F+ temp difference between the outside and air intake (inside the engine compartment). This is with the constricted inlet, covered front, etc. as sealed as it normally is, and after the run (stationary).

However, the second is more interesting one for our purposes. This shows a 7F+ difference at higher temp, *but* it was with the nose of the car completely taken off, and at speed. (I replaced the AC condenser on my car and drove it to the shop au natural). So the temperature gain with maximal air infiltration is at least 50% that with minimal air infiltration. Note that all this is post radiator, so what we are really measuring here is the temperature raise across the radiator.

Okay. I believe we can draw some conclusions. The first is that your air replacement model is very reasonable and is definitely at play. If the engine temperature is 185F but the engine compartment air is near ambient, the air must be being replaced frequently. In fact, I've seen this happen in reverse in real time, i.e. when I stop and see the temp of the engine compartment go up. [Hmmm... this also means that testing the hood temperature by hand is time dependent.]

However, that's not the only factor going on. It's also the case that we have a *heat flux* occurring as well, from the engine, through the radiator, to the flushed air. Unfortunately, the temperature is only a rough proxy measure for the heat flux. But even here we can make some tentative conclusions. The most important of which is that we should expect insulation changes temperatures *on the order of a few degrees*, but that's sufficient to significantly alter performance characteristics.

Now onto the mechanism. I'm proposing the insulation works by actually blocking heat flux, while you are proposing it could also be simply by blocking airflow. We could figure out precisely how much each factor contributes by putting a heat conductor in place instead of insulation. For example, wadded up aluminum foil. Something like that should transmit the heat but occupy the same space / block the same airflow.

Would you be up for an experiment to test it out?

[hayden55]

Either way it still works. Reducing ACH and insulating a house both save energy. ACH meaning air changes per hour. But basically sealing the envelope helps a lot just like insulation.
The big one being it seals off the top of the motor from air flow, but when you walk away from the car it doesn't cool down so quickly. Cruising down the road the sealing of the head with some insulation like i did more or less reduced the forced cooling over the head.
I agree though thats why i brought up my example with engine bay leaks earlier.

[Ecky]

I'm up for testing, but I 1) presently do not have any insulation batts, or know what I'd do with them if I didn't keep them in the engine bay, and 2) do not daily my MX-5, it's strictly an evenings and weekend car.

[Phase]

Engine temp after having car parked for 30 plus minutes while I went into the ski lodge. Definitely helping to retain some heat! I just need to attach the insulation to the hood versus laying it down. Hope my engine doesn’t over heat on my aggressive trips though. Insulation may be working a little too well?

[clydekgeorge]

Ceramic coated cylinders, pistons, and combustion chambers have been shown to increase fuel efficiency. I don't think they were ever able to get a durable cylinder coating, but pistons and chambers are still done sometimes

[Piotrsko]

Had my O-290 lycomming engine replated in something called nikasil or cera-chrome which, according to reports, made the cylinders resistant to wear by the aluminum pistons.