Engine insulation for hybrids to improve fuel economy

[Talos Woten]

Howdy all!

I've recently discovered a brand new ecomod that's not listed on the 65+ Vehicle Mods page, but should be. It's to better insulate the engine bay, in particular the hood. It's useful in cold weather and for short trips. Here's why.

WHY TO

Most ICE vehicles have problems generating too much heat, and thus have no insulation on the hood. They basically use it as a secondary radiator, which is quite effective when air is flowing over it. This has some downsides (a hot surface that might accidentally burn someone) but a lot of engineering upsides.

However, hybrids often have a problem maintaining operating temperature. They have smaller engines, which run leaner, and are turned off frequently to conserve fuel. Many will often have a "warm-up" cycle taking several minutes to get to operating temperature. This sometimes gets triggered after long coasts down hills if the temp drops too low. This warmup cycle is usually -15%+ from the typical mpg.

The real killer comes during winter and other cold weather. Now the engine has to remain on for long periods, and burn more fuel in general, just to maintain temperature. This fuel economy loss is often somewhere in the -10-25% range, depending on conditions.

So insulating the engine bay has two noticeable improvements:

1. It dramatically increase fuel economy during cold weather.
2. It modestly increases fuel economy by shortening warmup. This becomes more pronounced if you make frequent, short trips.

Insulating all the engine bay results in gains, but the hood is the place to focus on. The front is usually open for the radiator, the back is well insulated already to protect the cabin, and the bottom is where venting occurs. The hood is a large, flat, easily accessible surface, while the sides are curved and more difficult to get to. Also, heat rises, so we get more bang for our buck starting with the hood.

Let's describe how to do the hood and then talk about the sides after.

HOW TO

Now that we have the motivation, let's talk about how to do it. Raise your hood and look at the current hood insulation. In my case (of a 2009 Toyota Prius) it was a cardboard-like material that had stamped regions for stiffness. It was attached by plastic clips. Let's call this the hood shroud (because I don't know what it is actually called.)

Here we have our first decisions: what type of insulation to use, and how thick. I would recommend only three kinds of insulation, in order: specially made engine bay insulation (which you can buy stock), mineral wool mat like Cerablanket (2000F), and fiberglass bat (1200F). We can't use any material that might a) catch on fire, b) melt, c) scatter in the compartment (so no fill of any kind), or d) get wet / moldy / degrade.

Now, if you only see modest cold, the easiest course of action is just to fill the inside of the already existing hood shroud. My hood had cup-like sound-deadeners above the shroud, so you could just fill those in until flat, fill the shroud in until flat, and then replace as usual. You could probably get away with using the same plastic clips, but if you are concerned about weight you could put on real fasteners. Then seal the entire shroud outline with high temperature aluminum tape. That prevents any possible material leaking out from air eddies inside the compartment.

If instead you want to do some serious insulation, then drop the shroud down equally by some amount, say 1/4"-1/2". The limit on the shroud drop will be determined by how your kickstand rod is attached. Mine clips horizontally across the engine bay, so I had to lower the clip by adding a spacer and ensure the bar could still clear the shroud. If instead your bar clips out of the way of the shroud, the next obstruction will be whatever is on the top of your engine. You should leave at least 1" clearance from any stationary part, and 2" from any moving part.

I re-bored the holes where the plastic clips went and replaced them with M6 Rivnuts. Then I put a Cerablanket mat inside the shroud and cutout 7 holes: 1 for the kickstand clip, and 6 for the shroud holders. Finally I wrapped the whole thing like a burrito with aluminum reflective sheet. I don't think this is actually necessary from a thermal perspective, but I had it lying around, and it was another layer to prevent leakage of the insulation. Then I screwed the shroud back in place, about 1/4" lower that it was previously.

If you want to do the sides as well, you'll probably have to remove the fenders / front bumper, depending on the construction of your vehicle. Be mindful of any air curtain venting / other venting we don't want to obstruct. Most importantly, you want to install a new shroud over whatever insulation you install, and this has to be affixed securely. Here is where buying specially designed engine bay insulation is preferable, even though it's much more expensive. It's typically a sandwich of materials already adhesively compacted so it won't flake. So it can just be slapped down as a layer without a shroud needed.

COMMENTS

Combined with a grille-block, this modification is very effective. With a fully open stock grille (150 sq in) and hood shroud (R 0.2), the warmup time from ambient (80F) to operating (185F) was ~5 minutes / 3 miles. A safe grille block (80 sq in) reduces this to ~3.5 mins / 2 miles, and combined with hood insulation we get nearly 2 mins / ~1 mile. The FE bump over a 15 minute drive is +2%. This gets smaller the longer we drive, since the fuel saved over the few minute startup window gets averaged smaller and smaller. On the flip side, this should increase past +8% the colder the weather is, no matter how long we drive.

I've been focusing on aeromods for years and inadvertently found this ecomod along the way. In restrospect... it's an obvious mod to try. If the car engine gets too cold in cold weather, then insulating the bay better seems a reasonable route to go.

Moreover, I haven't seen this mod posted anywhere, so I thought I would share it so people would be aware of it. It really should be in the Warm Up mods section (maybe renamed to Warm Up / Cold Weather mods) of the main ecomod page. Moreover, I would expect that an Insulated Grille Block would be particularly useful for extreme cold weather performance, though I can't personally confirm that.

Apologies in advance if someone else has already posted this mod and I haven't found it yet. If so, please post in the comments so you get proper credit. Cheers.

[Phase]

I’ve definitely thought about this. My Ioniq blue has no insulation. Not even that cardboard like padding you mentioned. It’s just barely thin metal. The higher end ioniqs have some type of insulation under the hood actually. I think it’s for sound deadening though. Either way, I’ve thought about this for a while. Nice to know it actually works. Getting the engine to a warm temp is impossible in the winter time with city driving. Only on the interstate will it ever get warmed up, which on hybrids is also how it produces warm air. The first 15 minutes in single digit temperatures in winter going on ski trips around ski towns is horrible. I’ll be freezing cold and can see my breath in the car, and turning the heat on only blows cold air because the engine is so cold and turns off at every red light! It’s torture!!!

How safe is this for long road trips though? Sometimes I do drives they Arizona and Death Valley for hours and hours nonstop with 115 ambient temps in a black car. I had an engine overheat once in Georgia years ago on my old car so now I have ptsd about a too hot engine

[Talos Woten]

Hey Phase! Nice to meet you. (waves)

You bring up some good points. First, the hood shroud is also for sound deadening as well as insulation. That's also what the bubble stamped on the hood underside are for; to disrupt sound waves near the engine frequencies (as well as provide structural support). So I forgot to mention that a fortuitous side effect of engine bay insulation is reduced engine noise. I actually fully sound-proofed my car Champrius (I'm a huge audiophile) and every aeromod made the road noise lessen. It's got a super quiet sound stage now. ;-)

Surprisingly, the hood insulation has minimal impact on driving in hot conditions or for a long time. What's far more important is radiator airflow. So whether you are doing grille blocking / have a engine belly pan is a much a larger concern.

There are different ways to approach the safety issue. The optimal performance design is an inlet which is 40% the radiator surface area, diverging duct to the radiator, converging duct out of the radiator, and then an outlet +15% the size of the inlet. That is safe for super high performance vehicles like racing cars, and thus is an upper bound for passenger vehicles. On my car I've been using a 30% inlet with no ducting with no problems for years, under all kinds of adverse conditions.

The way to determine safety is to get a measurement tool of some sort and monitor coolant temperatures. I use Torque Pro with an ODB II reader, but really anything will do. Start with the stock configuration and you'll quickly discover the operating temperatures. On my 2009 Toyota Prius, it's 185-190F. I can put nearly any load on it in any (hot) temperatures, and the needle barely budges. (Cold is a whole other ball game.)

Then as you make modifications (like grille blocking), do test runs under loaded conditions and note the temperatures. What should happen is they remain reasonably stable until you hit a critical point, then you'll see large temperature swings. Then just back off to the last area / design that had stable temps.

To give you a ballpark, I discovered 30% radiator area grille block was completely safe under all loads, so that's what I kept it at. 21% was very reasonable and solid except for super extreme conditions (i.e. driving in Death Valley in a black car). I'm currently running a 12% grille block test, which I can get to jump +20F just under normal driving conditions (I'm testing other stuff, so I want a super sensitive inlet).

Don't take that as canon, though. You should measure it for your vehicle, with your mods, under your expected conditions. But the proof is in the radiator pudding. If the temperatures are stable under whatever scenario you are concerned with, then that's how one actually ultimately determines safety.

By the way, older cars used to have settings for the radiator inlet by season, i.e. "Summer", "Winter", and "Spring/Fall". If you do a lot of driving in a huge variety of conditions, you might benefit from a variable inlet and outlet system. Something near 40% / your max for Death Valley, and 10% / your min for cold weather or somesuch.

Hope this helps!

[Phase]

My Ioniq has active grill shutters so no need for engine grill block. They stay closed when they can, and open when they want air flow in them. The front belly pan has a few squat holes in it, which I figured was for oil changes and getting to engine areas at first, but I think some of them are also for air flow too. I’ve always debated on patching up the belly pan holes for better aero, but don’t know how much of a diff it would make.

The hood of the underside of my Ioniq is pure aluminum. So I def need to add some insulation. I’m also an audiophile. I added some kilmat and second skin thermal jute on the floors in the front of the car, and sound deadened the whole trunk and spare tire area. I just need to do underfloor of back seats and also the doors. In the winter, I can feel the cold air radiating thru the plastic on the side of the doors. Was also going to do the roof liner with heat insulation too some day

[Talos Woten]

Originally Posted by Phase

My Ioniq has active grill shutters so no need for engine grill block. They stay closed when they can, and open when they want air flow in them. The front belly pan has a few squat holes in it, which I figured was for oil changes and getting to engine areas at first, but I think some of them are also for air flow too. I’ve always debated on patching up the belly pan holes for better aero, but don’t know how much of a diff it would make.

The hood of the underside of my Ioniq is pure aluminum. So I def need to add some insulation. I’m also an audiophile. I added some kilmat and second skin thermal jute on the floors in the front of the car, and sound deadened the whole trunk and spare tire area. I just need to do underfloor of back seats and also the doors. In the winter, I can feel the cold air radiating thru the plastic on the side of the doors. Was also going to do the roof liner with heat insulation too some day

Ohhh... the luxury. If you have an active grille shutter system, then you have no worries doing whatever the hell you want to the engine bay. The radiator system will adjust automagically.

Belly pan holes are for multiple reasons: water drainage, debris clearing, servicing, visual inspection, aero, airflow, and resonance. Unless it's a choppy, flimsy piece of junk, you are probably better off not touching it. Some of the holes are probably there for exotic cases we don't normally think about, like sloughing snow from the engine compartment after driving through a drift or somesuch. Just my two sense.

Yah, Killmat made a huge difference when I put it in my car, especially the doors, wheel wells, spare wheel well, and engine wall. I also put some insulation in my doors when I did the Killmat, for exactly the same thermal leakage reason. Actually, I insulated the whole car, just for the comfort aspect.

If you do the insulation on the roof, consider adding a reflective radiant barrier first (with a gap), then normal resistive insulation underneath. It makes a -5F difference on a hot 90F day. I also painted my roof white, which while garish, also cools by -3F on a hot day (from a gray color). It would have a bigger delta from a black. I hear they make IR reflective paints nowadays, so that could also be an option if one wants both heat reflectance and aesthetic beauty. Personally, I don't care for the latter.

[Talos Woten]

Oh! I forgot to mention. When screwing in the shroud use washers at least as big as the plastic heads of what held it in before. In my case, that was huge honking 1 1/4" diameter washers. That distributes the force over a larger area and make the shroud less likely to tear / crack / come apart.

[Phase]

Originally Posted by Talos Woten

Ohhh... the luxury. If you have an active grille shutter system, then you have no worries doing whatever the hell you want to the engine bay. The radiator system will adjust automagically.

Belly pan holes are for multiple reasons: water drainage, debris clearing, servicing, visual inspection, aero, airflow, and resonance. Unless it's a choppy, flimsy piece of junk, you are probably better off not touching it. Some of the holes are probably there for exotic cases we don't normally think about, like sloughing snow from the engine compartment after driving through a drift or somesuch. Just my two sense.

Yah, Killmat made a huge difference when I put it in my car, especially the doors, wheel wells, spare wheel well, and engine wall. I also put some insulation in my doors when I did the Killmat, for exactly the same thermal leakage reason. Actually, I insulated the whole car, just for the comfort aspect.

If you do the insulation on the roof, consider adding a reflective radiant barrier first (with a gap), then normal resistive insulation underneath. It makes a -5F difference on a hot 90F day. I also painted my roof white, which while garish, also cools by -3F on a hot day (from a gray color). It would have a bigger delta from a black. I hear they make IR reflective paints nowadays, so that could also be an option if one wants both heat reflectance and aesthetic beauty. Personally, I don't care for the latter.

i have a ton of this stuff - https://www.secondskinaudio.com/heat.../heat-wave-pro

was going to put it in my roof liner. thick, insulating, blocks a little sound, and reflective

[Talos Woten]

Originally Posted by Phase

i have a ton of this stuff - https://www.secondskinaudio.com/heat.../heat-wave-pro

was going to put it in my roof liner. thick, insulating, blocks a little sound, and reflective

Perfect! That's a combo material which is exactly what you want. It does a little bit of everything in a thin package. If you have enough space (and material) you may want to double / triple up the layers. It adds enough thermal benefit to offset any weight detriment.

In my own case I found the insulation I put in actually had a tiny but measurable impact on my mpg (+0.4%), BUT only in the scenario where I was running the AC or heat near full blast. Otherwise, I could notice a difference in comfort but not on FE.

Hmmm... wait, that statement is inaccurate. *All* the thermal mods I made had that total impact, including the window vinyl, coloring roof white, and roof vent. So the insulation alone is some fraction of that... maybe half? I can't say for certain because while I did all the temperature measurements individually, I did the FE test as a batch after they were all done.

[mpgmike]

Reminds me of a historical piece, the Shell Oil 1959 Opel Rekord which Shell claimed achieved 376.59 MPG!!! The engine was a 4-cylinder with heavy spray-on insulation. Here are a couple links:

https://www.hemmings.com/stories/200...opel-uncovered

https://www.youtube.com/watch?v=EtR5WjwocBI

https://www.youtube.com/watch?v=EtR5WjwocBI

[Drifter]

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.

I suppose you could ceramic coat the exterior to slow the rate of heat loss between use.