Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Engine insulation for hybrids to improve fuel economy

Talos Woten · 06-07-2022, 12:00 PM

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:

It dramatically increase fuel economy during cold weather.
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.

06-07-2022, 12:00 PM	#1 (permalink)
Talos Woten Righteous Imaginarian Join Date: Jun 2022 Location: Aliquippa PA Posts: 107 Champrius v3.2 - '09 Toyota Prius 90 day: 58.73 mpg (US) Thanks: 7 Thanked 103 Times in 53 Posts	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: It dramatically increase fuel economy during cold weather. 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.