Engine insulation for hybrids to improve fuel economy
 

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.

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

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!

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. :D

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. :p

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.

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. :o

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

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.

https://www.secondskinaudio.com/heat-shield/


any opinion on whats the best product for engine insulation from this company?

Process of Elimination
 

The only ones you could use are the Radiant Shield and Firewall Ceramic. The others are only rated to 250F, which is too close to engine running temperature. I expect the Radiant Shield won't do much; you are looking for resistive insulation rather than reflective. That only leaves the Firewall Ceramic, which is rated to 400F.

Just FYI, I got a mat of mineral wool Cerablanket off Amazon rated to 2000F for a third of the price. If such things matter to you. :D

it says the mega block sheet goes to 1000f?

If you look closer down the Mega Block page, it says the foil shield is only good until 450F. That's consistent with the working temperature of melamine foam:
https://melaminefoamtech.com/melamine-foam/

It says it requires 2" clearance to go to 1000F, which is completely misleading advertising. Personally, I avoid any product that tries to oversell itself. But it's your purchase decision.

Is there a reason why you are limiting yourself to this particular company? Employee discount, or a great blackmail situation with the manager? :D

just good experience with their jute thermal insulation i used for the interior of the cabin. they have high quality products and good customer support

though they did try and push me hard to get their sound deadener, but i went with kilmat from amazon instead. second skin and dynamat have some of the elite products, but are overpriced. i dont mind trying something cheaper that works and has great reviews, hence why i chose kilmat over dynamat lol

duckduckgo.com/?q=basalt+fabric+engine+compartment+insulation&ia= web

That's 1472 F.

I remember members arguing about this.

I found this from 2009: Developed for winter commuting (but now I run 'em year around)
Frank Lee cut a sheet of cardboard and put it under his car hood and used 1" foil-faced foam in his truck.#23 11-30-2017

Daox posted Super fast warm up mod - Engine insulation in 2011.

The fifth post is Frank Lee sharing his older thread.

The argument I remember is that cardboard and coroplast have negligible insulative properties.

I also remember people putting fiberglass welding blankets under their hoods, but I will need to finish that thread and possibly look elsewhere later.

I use a fibreglass fire blanket and it holds up well in my CR-Z. It reduces the warmup time and retains engine heat for longer periods. Works great in tandem with an engine block heater too.

Thanks for all the helpful links, Xist! I thought I couldn't have been the first person to think of using more insulation in the engine bay. It seems like such a straightforward idea.

As for the insulation, it's often misunderstood about how it works. The R-Value of dry, non-circulating air is ~ 3.5 per inch. All it takes is a sealed envelope to achieve that, and that's how the vast majority of insulation works. It's how fur keeps animals warm, sleeping bags are disproportionately effective, aerogels offer insane protection, etc.

When calculating the effective temperature drop, we need to account for the static air gap as well as the actual material itself. So while a 3/16" of cardboard is only R 0.05ish, if it traps an inch of air then its really 3.5 or so.

This is significant when calculating the flux of heat through the surface. Metals have very high heat transmission properties; it's why stuff like copper and aluminum are used at heat sinks. The heat flux is a function of the temperature delta between source and sink. So the smaller one makes that (say, through insulation) then the smaller the thermal flux becomes. Optimally, one wants exactly enough insulation so that we have the engine operating temperature on one side and ambient on the other, with no flux through the surface.

By the way, there are more subtleties to insulation. We've been talking about resistive insulation, which blocks conduction. But there's also reflective insulation, which blocks radiation. And trapping air is effectively blocking convection.

Hope this helps!

Yah, this is a very easy experiment to perform for doubters. Most people have access to their coolant temp, and insulating the hood is a low cost and time test. As I like to say, "the proof is in the pudding." Go out and actually verify whether things work or not. :)

duckduckgo.com/?q=spray+insulation+with+vacuum+spheres&ia=web

resurrecting this. i am trying to figure out the best products to use for the hood. my car keeps staying sooo cold

Is it one piece, or two with an internal bracing gusset?

I believe two piece. I can take a pic in the morning tomorrow. It’s just bare metal

I insulated the bare hood/bonnet of my retired Peugeot 106 with dense rubber foam, with aluminum coating. Reflects and insulates both heat and noise.
More in this thread:
Piwoslaw's Peugeot 106 thread

Here’s what my hood looks like. It’s thin barely metal with some holes and open cavities. No insulation or anything. How should I go about this?

That looks like a worst case. With some internal bracing you might fish sheets of insulation in the gap.

It looks loke the best bet might be to take the hood off, turn it over, pour expanding foam in the cavity and scrape off the excess,

NOOO!!
You are hurting my eyes!!!
Images of naked hoods are disgusting:(

The premium models have a “ felt liner”

IIRC that was done to a single-cylinder 250cc air-cooled motorcycle engine by Yamaha. And that's a really good engine.

I'm about to just throw a piece of R16 or whatever in there. full fluff is 3.5" and is single digit dollars. I have a full upper and lower grill block on my prius and it still doesn't maintain temps, but it does do pretty well, it doesn't get those 50 degree swings like normal priuses. I think the insulation would be the last push it needed to be as good as it can get.

https://www.dynamat.com/products/hoodliner/

anyone know how this does for heat?

Damn boys i put R13 or whatever on the prius since i have a roll for my garage door i'm doing. Slapped it on and heat retention is massively improved. with a upper and lower grill block and a header air intake i'm basically back up to summer gas mileage around town with the automatic heat on as well.
i used a piece of 3.5" thick by 48" long and i think 18 or 24" thick. so like 4$ worth

I'm going to get some next time I roll past Graffiti Alley on River Road. I think they had a competitive knock-off product as well.

I've always been told if you can see the heat source use that shiny stuff, and if you can't to depend on the air gap. So think like an infrared light and metal objects. Instead of absorbing the infrared it'll bounce off more. But with that said i would just go with airgap on this one. Toyota oem engine bay insulation is just yellow fiberglass, albeit more compressed since its more for sound and not for heat retention.
I'll have to add a pic of my engine bay.
$20 warm air intake tube, $2 in foam, and $4 in insulation later and i have a car that can do 60mpgs in the winter with the heat on. I never have to worry about dropping below like 170s and the car won't go over 202, even the inverter temps are good.
Its seems so good when temps get above 60s again i will leave the insulation and just unplug the grills in order of what i need to maintain good inverter temps and what the cooling system for the engine can handle.

Talos you win bud. never in my life would i have ever thought to do this, but this is why i love the internet.

Not even lying I think this is like the most effective mod ive done personally. I may redo it and cover the whole hood area and leave a gap around the inverter.

I knew about this mod, but haven't actually insulated the hood. I've sealed the hood seams through with truck topper foam, works ok but the hood can be a challenge to pop sometimes since snow melts and freezes in the crack.

I've been thinking about doing something like this on my prius since as you say, warmup time is 3-4 miles with a grill block and lately my trips are both cold and short trips so should be a massive improvement.

I haven't seen any info on this idea before, but another concept I've had that's similar to this is oil pan and transmission heaters that run only when the engine is cold assuming you don't have an easy to access outlet to pre-heat the vehicle. The theory is to warm the engine up quicker to get it to operate more efficiently, but the extra load while code might offset any benefits. It could be possible to use the prius logic and run the heaters off a spare battery then once the engine is warm, recharge the battery back up off the engine, or take it inside to charge off the grid/battery bank for the house.

Anyway, sounds like I need to throw some fiberglass insulation over my engine and see how much of an effect it has short term to I work out a better long term version =).

You all using bagged insulation or typical house one sided?

The bagged stuff comes foil covered both sides or not, but the bag used to be semi puncture /rip proof

I planned to use just normal house fiberglass insulation for now just for testing. Remove the paper and there's no fire hazard. I know several people that have used the same process to build their own insulated wood stove pipe and it works quite well. I'm sure there's better products to use, but the top of the engine doesn't exactly get that hot, I mean as the OP post says, there's plastic on the hood, so it's not going to get over ~125C as most plastics don't go hotter than that. PVC is roughly 80C which is common cheap wire insulation. Higher grade engine bay wire is 105-125C rated and intended to be used right on the engine, like the injector harness.

I figured that's the easiest thing I have around that's basically fire proof and insulates.

the paper is fine. just put insulation side down. As said it doesn't get that hot in the engine bay. around the front and top of the engine you may get up to 150F air if you're lucky. Engine metal temps a bit higher but not a big deal. The big problem is water though, but my engine bay stays dry up top so figure it on your personal vehicle. Just don't touch the headers lol