Hi

Read in your papers the other day about a new UK law ( they are thinking of bringing in) regarding switching off the engine while stuck in traffic. Traffic wardens will be able to issue the penalties £20 spot fines if you leave your engine running while at rail crossings and town centers. Question is ? what happens to air conditioning systems, how long do they run for ( if at all) when the engine is turned off. ? Do we all go back to opening doors when the temps are in the 40c, thus giving problems to cyclists etc. I must say I am opened minded about the idea because of pollution etc, not sure if the sums add up to what in fact are the savings.

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Octy 2, 1.9 TDI PD Ambiente 4x4 Combi and Nissan Sunny 2.0 DLX, Company Octy 1.9 TDI Tour Combi.

There must be savings, because that is what the super-efficient cars do automatically.

You are right though; the AC goes out immediately the engine stops.

Here in the UK it will only be a problem one day a year

I thought it had been shown in tests that the "start-up" process on a car negates any benefit of actually switching the engine off when stuck in traffic.

Not only that - but in a traffic jam situation you tend to be "bumper-to-bumper".

All it will take is for one of those cars to not start again after being switched off and the nightmare begins.

I'm afraid I'm not open minded on this one - I think it is a bad idea.

I wonder if there is a definition of 'stuck in traffic' In parts of London you can often move 10 yards, then sit for two minutes, before getting a little bit further. I can imagine the wear and tear on starters/batteries, etc. If you take that the environmental impact of that into account it becomes a very bad idea.

Will be interesting as any small CO2 savings they are bleating on about will be negated by the CO2 required to make all those new starter motors and all those new cars wehn the engines wear out as they never get up to temperature.

To answer the original question, the actual cooling effect (or refrigerant effect) requires the A/C compressor to be functioning, so when the engine stops the effect is lost. However, in practice you'll find a residual amount of lower temps in the system that will keep cool air blowing for a while after the engine is switched off, and the outlet temps will slowly rise to match that of the outside ambient temperature. How long it takes to raise to a temp that is no longer comfortable simply depends on how hot is is outside. The hotter it is the quicker the temps will become to hot inside and you'll need to turn the engine back on.

If the car has an electric AC, than it is possible to run without the engine running. AC compressor in a normal gasoline engined car is always propelled by the car engine.

In a prius (for example) there is an electric AC. This one can be used without the gasoline engine running (as long as it has enough volts in the batteries )

Actually, no. Most cars have an electrically operated clutch on the aircon compressor. Ask a friend to toggle another car's aircon on/off, but not on the Octi, it's spun full time, IIRC, the switching is done internally.

I thought it had been shown in tests that the "start-up" process on a car negates any benefit of actually switching the engine off when stuck in traffic.

Obviously it depends on how long you're going to be switched off for. I've heard various figures quoted as to when it becomes more advantageous to switch off, from those who say that any time longer tahn 30 seconds is a saving to claims that it needs to be at least seven minutes. Obviously it depends what car you have, but I also suspect that some calculations are factoring-in or ignoring different knock-on effects.

Not only that - but in a traffic jam situation you tend to be "bumper-to-bumper".

All it will take is for one of those cars to not start again after being switched off and the nightmare begins.

Not to mention the occasional poor benighted soul who will stop in gear, forget that detail and then start up without depressing the clutch, thereby lurching into the car in front, causing a delay for inspection and possible exchange of details...

I'm afraid I'm not open minded on this one - I think it is a bad idea.

It's a typical idea from somebody whose thought process goes no deeper than "it's bad to have engines running without moving - therefore we must force people to turn them off". It's a knee-jerk suggestion from somebody hard-of-thinking who believes that they have to be seen to be doing something. Do we need another crappily-thought-out law to show us how bad "reaction legislation" can be? Can't we just look at the poll tax or the dangerous dogs act? And o how happy I am that at least some of the thought/research/publication of such a thoroughly cr@p idea was done by people paid by my taxes. :thumbdwn:

Such a law would either have to be utterly prescriptive, which would just bring traffic to a standstill (does simply waiting at a red traffic light count?) or allow so much discretion that it would be a next-to-pointless law.

Plus, with everybody having to turn on again as "traffic" started to move (I'm thinking of the roundabout at a local motorway junction here), that will delay traffic moving away, meaning less traffic moves with each cycle of traffic lights, meaning the traffic builds up leading to slower traffic flow, longer journey times, arguably some reduction in business/commerece efficiency, and more traffic sitting in slow moving traffic chucking out extraneous exhaust before they come to a stop and have to turn off.

Are they going to say any time you come to a stop? any time you know you'll be stopped for more than say two minutes, or any time you might have a "reasonable expectation" of being stopped for a certain length of time. Like how is that going to be judged? For example, there is a level crossing that I regularly use adjacent to a branchline station. Now, if I bowl up to the crossing and the barriers are down, and there's a fast train going through, I know that in about 15 seconds time, the barriers will be up and I'll be on my way. Even if it's a big goods train going through, I'm likely to be moving within a minute. But only if that's the only train going through. Sometimes the train goes through and the barriers stay down. It's not unusual to get a "double header" or "triple header", where a fast train comes through in one direction, then a stopping train in the other direction pulls into the station, and then a stopping train in the other direction. I've even occasionally sat there whilst four trains have passed through, and once even five. Oh, plus the occasional times it seems to go wrong (usually at weekends) and just stay shut for 10 minutes after a train has been with no other train subsequently appearing. How the hell is one supposed to gauge that? (Although I suppose the chance of a traffic warden in a country village is minimal, especially on a weekend. Although equally I might say that the chance is minimal until the local council realise what a nice little earner it will be...)

Apologies to the original poster for rather ranting off-topic to the original question.

No need to apologize sir, I totally agree with your sentiments. I'm pretty convinced it's just another tax grab concept which has absolutely nothing to do with green issues. After all, have they said what the cash they make is going to be used for? R&D into new engine technologies? I don't think so.

IIRC in Germany you have to turn the engine off if stopped at a level crossing etc. for any length of time.

in spain you had to turn your car off if parked up, which seemed alright to me

Bad Bad Bad Idea, my starter is a lil weak once the engine is up to temp.

Would mean my battery would be dead by time i got out of sheffield town centre!