Improving the AC system's efficiency

[EcoCivic]

Originally Posted by oldtamiyaphile

Besides, I've never heard of an exhaust manifold catching fire

I have. I read about an incident where there was a front end collision with a newer Mercedes, a refrigerant like ruptured, refrigerant and oil sprayed on the exhaust, and the car burned to the ground. I wish I could find that link. However, I did find this video where a car actually burns down due to flammable refrigerant:

https://www.youtube.com/watch?v=P19sG_SOPjY

[MeteorGray]

I don't think I would choose a refrigerant that could turn the air conditioning system into a blow torch. Besides, my car is mostly used at highway speeds, and I can't really tell that the air conditioner is reducing my fuel mileage on any given trip. I know it does because it takes energy to turn the compressor, but the other prevailing factors are much more noticeable: speed, wind, traffic congestion, etc. I live in the deep south of the USA, so my air conditioner is used a lot.

One thing I have done to help the air conditioner and cooling the engine bay in general is to install hood vents. They let a lot of the trapped air out.

[Piotrsko]

You all know that propane has been used for maybe 20 years as a enviro friendly household refrigeration gas replacing R12?

We recently had a Tesla attack a power pole and catch fire. Does that mean we have to consider power poles dangerous?

[EcoCivic]

Originally Posted by Piotrsko

You all know that propane has been used for maybe 20 years as a enviro friendly household refrigeration gas replacing R12?

We recently had a Tesla attack a power pole and catch fire. Does that mean we have to consider power poles dangerous?

An environmentally friendly household refrigeration gas. I used the same Envirosafe Industrial R134A replacement in my refrigerator about 2 years ago after I changed the compressor, and I used R290 to recharge a window AC unit that originally used R22 after the condenser was damaged by careless movers and it lost it's charge. Both systems work perfectly, I don't consider either dangerous.

However, a refrigerator that holds about as much gas as a few lighters, is generally away from ignition sources, and is well protected sitting in the kitchen is a LOT different than a car that holds a significant amount of gas in a very unprotected location (the condenser at the very front of the car) and is relatively likely to crash with plenty of ignition sources in that area. Damaged wiring, hot engine components, and exhaust pipes are all potential ignition sources in an accident.

[jeff5may]

Ok so the general consensus is that filling up your automotive air conditioning system introduces the risk of fire in case the sealed system is compromised. Some agencies are advocating the move away from environmentally unfriendly refrigerant, but the manufacturers have not found any practically viable natural substitutes they're willing to stand behind. The small appliance industry has begun to adopt the stuff mentioned in this discussion, but not the automotive industry. This doesn't mean they aren't going in that direction, as a possible candidate, r1234, is somewhere in the flammable range.

AFAIK, it is not illegal for a private owner to modify the air conditioning system in this manner. The risk is not merely theoretical, it's been experimentally and practically demonstrated. For a stuntman or a street racer, please use non-flammable refrigerant. For the other 99.9 percent, it's a personal decision. I would recommend buying a glovebox fire extinguisher for everyone who drives a car anyway, just like jumper cables and road flares.

That being said, the main issue with the retrofit is the system refrigerant charge. This will vary depending on a few factors: absolutely depending on the gas or blend being charged, as well as the metering device employed; more subjectively depending on end goals. Maximum system efficiency and maximum system capacity are achieved by different amounts of charge.

[Ecky]

Originally Posted by MeteorGray

I don't think I would choose a refrigerant that could turn the air conditioning system into a blow torch. Besides, my car is mostly used at highway speeds, and I can't really tell that the air conditioner is reducing my fuel mileage on any given trip. I know it does because it takes energy to turn the compressor, but the other prevailing factors are much more noticeable: speed, wind, traffic congestion, etc. I live in the deep south of the USA, so my air conditioner is used a lot.

One thing I have done to help the air conditioner and cooling the engine bay in general is to install hood vents. They let a lot of the trapped air out.

In my car, the loss can be 15-20mpg on the high end. Mine you these are cases where that's only 15-20% total fuel economy, and really blasting the A/C, but it's non-trivial.

[EcoCivic]

Originally Posted by Ecky

In my car, the loss can be 15-20mpg on the high end. Mine you these are cases where that's only 15-20% total fuel economy, and really blasting the A/C, but it's non-trivial.

Saving even 10% power running the AC would be significant IMO, and I'm pretty sure the efficiency gain would be larger than that. Not only does the car cool down quicker and the compressor runs less, the compressor takes less power while it is running because the head pressure is lower.

[jeff5may]

Originally Posted by EcoCivic

Saving even 10% power running the AC would be significant IMO, and I'm pretty sure the efficiency gain would be larger than that. Not only does the car cool down quicker and the compressor runs less, the compressor takes less power while it is running because the head pressure is lower.

From this perspective, the question lies in the amount robbed from the available motive force. With less power applied to move the vehicle, the cooling system steals a larger piece. Maximizing the leverage of the phase change circuit will net only a few percentage points of efficiency. After that, the level of comfort will be affected.

Running lower system pressures and or compression ratios will decrease power draw. This comes at the expense of reduced cooling system capacity. Not much of an expense in less tropical climates, but the closer we get to the equator, or the middle of the summer, the more insufficient the reduction in capacity becomes at satisfying comfort. This effect has a way of rearing its head on a holiday weekend in gridlocked traffic.

Hardcore options are plenty, but it all boils down to a personal decision: where does the need for comfort outweigh the performance? Sweating like a pig during the rare occasion pays large dividends the rest of the time.

[EcoCivic]

Originally Posted by jeff5may

From this perspective, the question lies in the amount robbed from the available motive force. With less power applied to move the vehicle, the cooling system steals a larger piece. Maximizing the leverage of the phase change circuit will net only a few percentage points of efficiency. After that, the level of comfort will be affected.

Running lower system pressures and or compression ratios will decrease power draw. This comes at the expense of reduced cooling system capacity. Not much of an expense in less tropical climates, but the closer we get to the equator, or the middle of the summer, the more insufficient the reduction in capacity becomes at satisfying comfort. This effect has a way of rearing its head on a holiday weekend in gridlocked traffic.

Hardcore options are plenty, but it all boils down to a personal decision: where does the need for comfort outweigh the performance? Sweating like a pig during the rare occasion pays large dividends the rest of the time.

It is not necessarily true that reducing compression ratios and power draw will decrease the system's performance. By that logic, blocking the condenser's airflow would help the system's performance by increasing power draw and compression ratio.

I noticed that when I converted to the Envirosafe refrigerant, the AC worked better when stuck in traffic, and the head pressure is lower too, I think by about 20 PSI if I remember correctly.

[jeff5may]

If you are operating the system within the envelope of r134a, it's not too hard to beat. Propane especially is more forgiving of superheat because it has higher specific heat than r134a.