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|16th October 2008, 17:52||#196|
Join Date: Jul 2006
Specific Fuel Consumption
When we use the word ‘specific’ in an engineering context, we’re describing something in the light of a comparison. So, ‘specific power’ is probably better known to you as ‘kilowatts per litre’, or ‘hp per cubic inch’. Simply, it’s the peak power divided by the engine capacity.
Specific fuel consumption is similar. It’s the amount of fuel consumed, divided by the power being produced.
So it could be expressed in litres of fuel divided by the kilowatts developed. In fact, the fuel quantity is normally described as a weight in grams or pounds, so Specific Fuel Consumption is expressed as:
Grams (per minute) per kilowatt (hour) - or just grams/kilowatt hour (gm/kWH)
Pounds (per hour) per horsepower (hour) – or just pounds/horsepower (lb/hp)
Sometimes you’ll find other units being used as well. However, don’t get too hung up on the units – remember, they all express fuel divided by power.
Firstly, why should the SFC be lowest at middle revs? Or, to put this another way, what causes an increase in fuel used per kW at both low and high revs?
At low revs, SFC suffers because there’s increased time for the heat of combustion to escape through the walls of the cylinders and so not do useful work. At higher engine speeds, the frictional loses of the engine rise alarmingly (especially in this case with 12 cylinders!) and so the energy of combustion is again being wasted, this time in heating the oil.
There’s another reason that SFC is lowest at ‘middle’ rpm. Because the engine is tuned to develop best cylinder filling (ie to produce best torque) at middle revs, the engine’s breathing is at highest efficiency at these speeds. But don’t fall into the trap of saying that SFC is always at its best at peak torque – that’s not usually the case.
But the real trouble with diagrams like the one above is that in many ways, they’re irrelevant to real-world fuel consumption. Why? Because these graphs are drawn for full throttle! So if the engine is powering a racing ski-boat travelling constantly at full load, then yes, the shown SFC data is all well and good. But what happens at part throttle, as occurs in nearly all normal car use?Well, then, the situation is very different! And the trouble is, the SFC figures are always much worse... [/FONT]
This graph shows what happens at lighter engine loads – it’s from a Repco manual for “a typical four cylinder” engine. The SFC is expressed this time in pounds per horsepower hour – but as we said earlier, it doesn’t matter what units are used.At 100 percent load (ie wide open throttle) this engine has a minimum SFC of 0.43 – see the bottom curve. As we by now expect, at both lower and higher revs that this, the SFC rises. But have a look at what happens at 50 per cent load! The SFC results at half load and 1000 rpm (ie idle) doesn’t matter much (when would you be in that situation?) but at 2000 rpm, the SFC has gone up by 13 per cent. At 4000 rpm, it’s gone up by just under 30 per cent!And keep in mind that in normal use, even 50 per cent is a lot of throttle. A more frequently used load is 25 per cent. At 25 per cent load, the SFC at 2000 rpm has risen by a massive 117 per cent over that achieved at full load! You can also see from the shape of the 25 per cent load curve, BSFC is even more heavily influenced than ever by the rpm being used.
So what accounts for this terrible decrease in SFC at just the throttle openings the engine will be used at most often? ‘Throttle’ is the key word here – as the engine is increasingly throttled, it has to work harder and harder at drawing air past the throttle blade. This is the reason that there is a measurable vacuum after the throttle blade – the engine is trying to drag in more air than it is being permitted to. Each time a piston is descending on the intake stroke, it’s having to do this extra work. Working internally hard as a vacuum pump means there’s less power available at the flywheel...This work against the throttle restriction is referred to as ‘pumping losses’.
When You Close the Throttle..thats the catch...<i hope all the graphs are properly attatched
actually for an average 1000kg sedan we need @7 to 9 bhp to overcome the wind resistence and @6 to 8 bhp to over come the rolling resistence.. in short we need @ 20 to 22 to the max to propel the one tonner at 100 kmph .. at full throttle ... rest is pure execess!!!!!!
Last edited by TURBOSAM : 16th October 2008 at 17:59.
|17th October 2008, 09:41||#197|
Join Date: Jan 2008
Location: New Delhi
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|17th October 2008, 10:00||#198|
Senior - BHPian
Join Date: Nov 2007
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Last edited by Guna : 17th October 2008 at 10:02.
|21st November 2008, 13:15||#199|
Join Date: Jul 2006
the formula is 1/2*p*v squared *Acd..
A is frontal area
p is density of air v iS the velocity of the vele relative toair ..
so drag is +dynamic pressure*projected area*coefficient of drag<cd>
it can be seen from the expression that the drag depends upon the the product ofthe frontal area and the coefficient of drag ..
and not just the drag coefficient ...
lower drag coefficient doesnot necesorily mean lower drag
VW micro panel van<combi>has cd of 0.45 which is lesser than contemp jag racer d type which was 0.49
but the drag is far more as the frontal area was much more 1.04 vs 0.59 the PRODUCT OF CD AND THE FRONTAL AREA IS THE DRAG... BUT IT HAS BECOME COMMON FOR THE CAR MFG AS WELL AS THE EXPERTS TO <MISS>QUOTE JUST THE CD VALUE
|25th November 2008, 15:39||#200|
Join Date: Dec 2005
Thanked: 3 Times
http://www.team-bhp.com/forum/techni...etrol-etc.html (Factors affecting Thermal eff vs FE and diesel vs petrol etc)
The sfc is affected more due to the air fuel ratios. As loss of heat through liner walls is quite low for such high speed engines (about 5 ~10 % on average) and the change in the loss of heat through liners is insignificant as compared to loss through the air.
At low rpm, the air fuel ratio is too lean and hence lot of air is wasted without being utilised in generating power. At higher loads, the air fuel ratio is on the richer side and hence the thermal efficiency is less as fuel is not properly burnt.
The sfc curve is flatter for turbocharged engines as the free running turbochager is more load dependent than rpm.
In the curve shown the sfc is least when at the point where "y" value is least and torque is max where "x" value is max.
|6th June 2010, 11:28||#201|
Join Date: Jul 2006
for non stratified petrol engines .the ratio of 14.7:1 is mandatory for regular fuel.
15.3 to 15 .7 for 101 octane fuel..
so the petrol engines are not running lean at low rpm and rich at high..... or the engines dont pass emission norms.
Note from Mod : Please continue the discussion in our Article on Maximum Fuel Efficiency (ARTICLE: How to get the maximum Fuel Efficiency)
Last edited by GTO : 7th June 2010 at 11:25. Reason: Adding thread closure note
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