on modern cars does the useage of lights make a massive difference on mpg.
Those of you who do the same journey daily, is there a massive diffenence, assuming you actually turn your lights during the winter months, or does the usage of air conditioning in the summer months, more than compensate.
And what about the radio- i imagine its neglibable
Finally, are all recorded mpg's calculated with or without electrical items in use ??
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I asked a similar question about a week ago and I was advised that headlights on a notional 14 hour journey would use 1/2 litre (50p) at most. Therefore less than 4p per hour so not really noticaeble.
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Aircon would use more power than headlights, I'd expect, though it does depend how much work the air con is doing.
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If fuel is not supplied to engine, how will it run then?
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If the car is moving and in gear with foot off throttle then the engine is driven by the wheels. Makes sense for no fuel to go to the engine in this situation.
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I'm really confused here :(
If wheels drive engine that'll obviously cause wear & tear on engine (as it was designed work in opposite manner). When we see the "Downhill - use lower gear", I think it suggests using lower gear because engine rev will rise in lower gear so that it will still cause the engine to drive wheels. Obviously when wheels turn the engine, you'll lose the engine braking advantage.
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>>obviously cause wear & tear on engine
Not really - as long as the engine is turning, so is its oil pump - so everything is still being correctly lubricated.
>>"Downhill - use lower gear"
The idea of this is so that you don't need to use your brakes as much, preventing them from overheating. Effectively, you are usin the friction of the engine as a brake.
In normal driving, the lower gears allow the engine to have more "authority" over the motion of the car - you get the strongest acceleration in the lower gears. You also get the strongest engine braking in the lower gears, hence the recommendation.
Number_Cruncher
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If wheels drive engine that'll obviously cause wear & tear on engine (as it was designed work in opposite manner).
No reason for it to cause extra wear - the pistons/valves/crankshaft etc. still get their oil feed from the oil pump, ditto gearbox/propshaft (if rwd) from their oil bath. Clutches are agnostic .
When we see the "Downhill - use lower gear", I think it suggests using lower gear because engine rev will rise in lower gear so that it will still cause the engine to drive wheels.
No - the wheels are driving the engine. The implication when you see that sign on a downgrade is that you'll be foot-off throttle making use of engine braking.
Obviously when wheels turn the engine, you'll lose the engine braking advantage
No - the exact opposite!
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Obviously when wheels turn the engine you'll lose the engine braking advantage.
That *is* engine braking!
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>>If fuel is not supplied to engine, how will it run then?
During overrun fuel cut off, the engine isn't running as such. The geartrain is effectively being used backwards, taking energy from the motion of the car , using it to overcome the friction in the engine.
Number_Cruncher
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I see. That has cleared something up for me too!
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04 Alfa Romeo 156 SW JTD 20v - Loving it.
Edited by Shaz {p} on 18/01/2008 at 13:03
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Idling the engine in neutral obviously does use some fuel. If going down hill in gear on overrun really does use no fuel, then by definition that is the more economical.
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No, only sometimes.
If you are rolling down a hill and will have to stop at the bottom then leave it in gear.
If you are rolling down a hill and will need to continue on, then rolling in nuetral is better.
I'm not sure if I'm explaining it very well, the difference is that out of gear you will pick up more speed/momentum and travel further for a given amount of fuel. Its not just the fuel consumed its also the distance travelled that needs to be considered.
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on modern cars does the useage of lights make a massive difference on mpg.
Negligable. Say that your lights are using 150w, total (headlamps, side lights, dash, tail, numberplate). Your engine is producing (say) 65KW!
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Re: electrical items effect on mpg.
To get an indication of which electrical items use the most power have a look in your driver's handbook at the detailed fuse ratings (or on the fusebox cover).
The items which use the most power and thus effect the mpg the most are generally the ones with the highest fuse ratings that are used for the longest periods.
You will find the biggest consumers of electricity are the electrical heaters.
On my car I have a 25A fuse for heated seats, a 25A for the glow plugs, a 20A for the heated rear window (plus rear inside lights), a 20A for cigarette lighter (plus radio).
All these heaters have a timer.
Each headlight has a separate 7.5A fuse for dipped and high beam.
Over exactly the same urban journey of 20 minutes twice a day I see a drop in mpg in the winter of around 10% which I attribute to longer engine warm up time and use of heated seats. (I have lights on all year round, negligable summer a/c use, a cold winter, plus winter tyres.)
I would say that manufacturer's official MPGs are recorded with "normal light electrical load". I know for a fact that in early 90's Land Rover 56mph and 75mph were officially measured on Spanish motorways. Certain times of year were chosen for preferential atmospheric conditions (not too hot/ cold). Close checks were made of tyre pressures etc with calibrated equipment.
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I'd agree with that - I seem to get about 10% more miles per gallon in summer than in winter but if I'm on my own [ ie only the headlights and not the radio on ] the difference is about 5%. Some of this will be down to the atmospheric conditions, but the principle is clear...
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Thanks guys - learned a lot today :)
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FT - you might have a 65Kw engine, but most normal cars (not talking about tiny underpowered ones, or ones with huge power to weight ratio) use about 20%-25% of total power when cruising in top gear. That said, the wattage of the bulbs is still insignificant.
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FT - you might have a 65Kw engine
I would guesstimate that most car engines generate 65KW (or more) at cruising speed.
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65Kw = about 85 bhp. That's roughly the output of a 1.4 petrol, I'd guess, though on older cars it might equate to nearer 1.6-1.8.
I'd be very surprised if a 1.4 was using more than 30% of max power at 70 mph. Aircraft piston engines are used at 75% or more most of the time, and there's the problem when using a car/bike engine for an aircraft. They can't handle the workload.
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I'd be very surprised if a 1.4 was using more than 30% of max power at 70 mph.
So would I - however - the power used to drive the car is not equal to the power generated by the engine. Around 3/4 of the power generated is lost (wasted).
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>>Around 3/4 of the power generated is lost (wasted).
Yes, that's true, but this heat power isn't included in the measurement of brake power - brake power is the mechanically available power, not the total power consumed.
Number_Cruncher
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- brake power is the mechanically available power not the total power consumed.
Yes - measured "at the brake". However, I have not (AFAIR) referred to this.
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>>I have not (AFAIR) referred to this.
Yes, you have, unwittingly perhaps.
>>the power used to drive the car is not equal to the power generated by the engine
The only losses between the power generated by the engine and the wheels are the friction losses in the driveline, and the rolling resistance of the tyres.
If cars really lost a significant proprtion of their power between engine and roadwheel, the transmission would need as big a radiator as the engine itself!
Number_Cruncher
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>>The only losses between the power generated by the engine and the wheels are thefriction losses in the driveline and the rolling resistance of the tyres.
This is also heat.
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The friction causes the heat.
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>>This is also heat.
And is also tiny. Not something to worry about, cetainly not any appreciable proportion of engine brake power.
Number_Cruncher
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At 70 mph, a mk IV Astra requires about 21kW to overcome all drag - so, if cruiising, this is all that's needed. If it's an 8v 1.6, the engine at this speed in 4th gear can produce about 59kW if required, thus leaving 38kW available for acceleration.
Number_Cruncher
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If headlights (and all the smaller lights) use 180 watts of power, that's 180 joules per second, thus 648,000 joules per hour.
If Diesel motor fuel = 38,657,950 joules/litre*, we can see that the lights use the same amount of energy as contained in a litre of diesel every 59 hours. We now need to know the efficiency of converting the fuel to power (37%*) and the power to electricty (70%*).
This gives 15 hours per litre, which sounds poor to me.
* - figures off other internet sites
Edited by Hamsafar on 18/01/2008 at 14:01
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^^ This sounds about right - 6.6p /hour.
A car travelling at 60 mph for 1 hour and 40 mpg will use £6.82 @ £1 per litre.
So headlights would consume around 1% in this case.
Edited by Billy Whizz on 18/01/2008 at 14:18
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I believe that using lots of in-gear overrun in town reduces the fuel consumption of my early-90s Escort 16v. Used to get around 25 mpg, but having to use a lot of overrun to prevent the engine stalling seems to get it closer to 30.
Of course someone is now sure to tell me that my car predates the fuel cutoff on overrun over 2,000 rpm engine management feature, and that I am just imagining the reduction in fuel consumption. So what though, if it keeps me smug?
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>>my car predates
Oddly enough Lud, I think your venerable transport does have this feature.
Number_Cruncher
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I remember fuel cut-off on overrun features on carburretor engines tears and years ago. The throttle flap had a spring-loaded plate which opened under vacuum, letting air in so reducing the suck at the jet. I'm sure there were electric cut-offs too - it's not very difficult to achieve is it?
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>>The throttle flap had a spring-loaded plate which opened under vacuum, letting air in so reducing the suck at the jet.
These weren't fuel cut-off devices as such - all they did was reduce the amount of fuel drawn in on everrun. If they weren't there, you would have greater than idle levels of "suck", and a hugely over-rich mixture.
They were also problematic - lots of the springs went weak, and made engines idle very erratically. Ugh!
Number_Cruncher
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I remember fuel cut-off on overrun features on carburretor engines tears and years ago.
Was this a typo, or a rich addition to our Shakesperian tongue? Whichever though, I shall file away for future use.
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hxj
The crucial wording there is 'out of gear'. In that case the engine drops to idle, and needs fuel. No question all the modern Vx cars (like virtually everything else on the market) have overrun fuel shut off above about 1500 rev/min. You can see the instantaneous fuel consumption display goes to 999.9 when you lift off.
JS
Edited by John S on 18/01/2008 at 19:33
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Thanks for the replies, all. Makes for an interesting read.
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