Go Back   Team-BHP > Under the Hood > Technical Stuff


Reply
 
LinkBack Thread Tools Search this Thread
Old 30th August 2011, 22:33   #1
BHPian
 
Join Date: Jan 2009
Location: Hyderabad/Mysor
Posts: 127
Thanked: 19 Times
Default Questions about fuel consumption at engine level

I'm starting this thread because after a lot of searching around on the thread, I found a lot of replies regarding getting the best fuel economy at various speeds but little about actual fuel consumption at the engine level. If this specific topic has been covered else where, please let me know and close this thread.

I've posted on a few threads relating to fuel consumption and I find that my knowledge on the exact mechanical process is rather limited. Let me draw up a base scenario before I start with my questions.

The engine is a generic normally aspirated 1.2liter petrol engine controlled by an ECU. We won't talk about the gearbox, differential, tyres and the myriad of losses and complexities that come along with them just yet.

Here's how I understand the process.
I believe that the whole point of the ECU is to achieve complete fuel consumption. That means that the petrol to air mixture needs to be just right. Now that would have been simple if it weren't for varying fuel quality and/or composition.

Ok, step by step, here's a rough outline to show how far my understanding goes.
1. Driver steps on the gas pedal.
2. The throttle opens up, drawing more air into the engine.
3. The o2 sensor notices this change and relays this information to the engine.
4. The ECU compensates for this increased oxygen content by increasing the amount of fuel being pumped, thus maintaining the air-fuel mixture at the appropriate ratio.
5. This also increases the piston speed, and since the engine moves up the rev band.

A few miscellaneous points.
At idle, the engine still has some amount of air coming in to let the engine idle.
Also, minor differences in fuel quality are also taken care of by this feedback loop.

Now for the questions
1. Does compression ratio have any effect on this?
2. Does a large-bore short-stroke engine have any advantages over a small-bore long-stroke engine? At the engine level, of course.
3. How do engine revs affect this? Is there any direct effect? My assumption is that since incoming air is more or less equal, there shouldn't be any major difference in fuel consumption.

Lets add in the gearbox and the rest of the transmission. Generic 5 speed box with progressive ratios.
I'll just post a series of theoretical scenarios. Which is better and why?
1. 50% throttle 2500rpm 80kph 4th gear OR
50% throttle 2000rpm 80kph 5th gear

2. 50% throttle 2500rpm 80kph 4th gear OR
40% throttle 2000rpm 80kph 5th gear

3. 40% throttle 2500rpm 80kph 4th gear OR
50% throttle 2000rpm 80kph 5th gear

I think that all three scenarios are possible in real life.



Now, for the turbocharger. Superchargers make life complicated. So lets stick with turbos.
Now, we know that a turbo works by drawing in more air into the engine by using energy from exhaust gases.
But if more air is being drawn into the engine, won't the ECU be forced to compensate by pumping in more fuel, thus increasing fuel consumption?


Here's a little theorem I've come up with. Tell me if they are true or false.

Since a liter of petrol has a limited and more or less constant energy density, and since air fuel ratio has to be maintained constant, the fuel consumption of an engine is inversely proportional to it's energy efficiency.

Here's an example. Two cars, same weight, same drag coefficient, same rolling resistance, both with a CVT capable of infinite ratios and lossless transmission.
Car 1 has a 3.0liter V6 running at 30% efficiency (rpm doesn't matter)
Car 2 has a 1.2liter I4 running at 30% efficiency (rpm doesn't matter)

The CVT is tuned on each car separately to have the cars run at 80kph. Then both cars must have equal fuel economy. True or False? And why?

What am I missing out here? Please correct me where I'm wrong. Thanks in advance.



PS. And what exactly are the brake specific fuel consumption graphs? Can they be applied to real life?
nukeblitz is offline   Reply With Quote

Old 30th August 2011, 23:44   #2
Senior - BHPian
 
oxyzen's Avatar
 
Join Date: Feb 2010
Location: Not fixed.
Posts: 1,018
Thanked: 214 Times
Default Re: Questions about fuel consumption at engine level

A few hours of study on the basics of IC engine should solve most of your queries.

Quote:
Originally Posted by nukeblitz View Post
Ok, step by step, here's a rough outline to show how far my understanding goes.

3. The o2 sensor notices this change and relays this information to the engine.
4. The ECU compensates for this increased oxygen content by increasing the amount of fuel being pumped, thus maintaining the air-fuel mixture at the appropriate ratio.
Not exactly. O2 sensor is basically for feedback. The amount of injection depends on air pressure and temperature in the intake manifold.

Quote:
Originally Posted by nukeblitz View Post
Does compression ratio have any effect on this?
Think compression ratio as - more is the compression ratio, better is the heat to work conversion. Better is the efficiency, more is the torque.
But in reality, with increase in CR, the knocking tendency increases, hence ignition timing is delayed to compensate it. As a result you don't really get what you expected for.


Quote:
Originally Posted by nukeblitz View Post
How do engine revs affect this? Is there any direct effect? My assumption is that since incoming air is more or less equal, there shouldn't be any major difference in fuel consumption.
More is the engine rev, more is the number of suction strokes per minute. Hence more fuel consumption.

Last edited by oxyzen : 30th August 2011 at 23:45.
oxyzen is offline   Reply With Quote
Old 1st September 2011, 16:53   #3
Team-BHP Support
 
Rehaan's Avatar
 
Join Date: Feb 2004
Location: Bombay
Posts: 19,090
Thanked: 9,627 Times
Default Re: Questions about fuel consumption at engine level

Quote:
Originally Posted by nukeblitz View Post
2. The throttle opens up, drawing more air into the engine.
3. The o2 sensor notices this change and relays this information to the engine.
Theres many sensors that work together, including the throttle position sensor.

Read this thread for a better understanding :
http://www.team-bhp.com/forum/techni...u-ecm-ems.html (TechSpec® : Understanding On-Board Diagnostics, also known as ECU, ECM, EMS)


Quote:
Originally Posted by nukeblitz View Post
2. Does a large-bore short-stroke engine have any advantages over a small-bore long-stroke engine? At the engine level, of course.
Read this thread :
Power characteristics of long / short stroke engines? (Power characteristics of long and short stroke engines: Cast in stone?)


Quote:
Originally Posted by nukeblitz View Post
But if more air is being drawn into the engine, won't the ECU be forced to compensate by pumping in more fuel, thus increasing fuel consumption?
Efficiency is the key word here. Internal combustion engines are far from efficient, and therefore any way to improve upon that inefficiency (aka minimize the losses) helps the performance overall.


Quote:
Originally Posted by nukeblitz View Post
Since a liter of petrol has a limited and more or less constant energy density, and since air fuel ratio has to be maintained constant, the fuel consumption of an engine is inversely proportional to it's energy efficiency.
I'd say true.


Quote:
Originally Posted by nukeblitz View Post
Here's an example. Two cars, same weight, same drag coefficient, same rolling resistance, both with a CVT capable of infinite ratios and lossless transmission.
Car 1 has a 3.0liter V6 running at 30% efficiency (rpm doesn't matter)
Car 2 has a 1.2liter I4 running at 30% efficiency (rpm doesn't matter)

The CVT is tuned on each car separately to have the cars run at 80kph. Then both cars must have equal fuel economy. True or False? And why?
It is possible that they could return the same fuel efficiency. I guess whether they do or not could depend on engine design and other factors(?).

Quote:
Originally Posted by nukeblitz View Post
PS. And what exactly are the brake specific fuel consumption graphs? Can they be applied to real life?
These graphs tell you exactly how much fuel the engine uses at each level of power(/rpm) for that engine

cya
R
Rehaan is offline   Reply With Quote
Old 1st September 2011, 22:30   #4
BHPian
 
amit_purohit20's Avatar
 
Join Date: May 2006
Location: mumbai
Posts: 447
Thanked: 268 Times
Default Re: Questions about fuel consumption at engine level

Quote:
Here's an example. Two cars, same weight, same drag coefficient, same rolling resistance, both with a CVT capable of infinite ratios and lossless transmission.
Car 1 has a 3.0liter V6 running at 30% efficiency (rpm doesn't matter)
Car 2 has a 1.2liter I4 running at 30% efficiency (rpm doesn't matter)
I do not agree with this because a 3 litre engine will have more swept volume than 1.2 litre, that means that the amount of charge will also be more in 3 litre engine. As the charge is more by volume and to maintain the proper Air/Fuel ratio the fuel quantity in that charge will also be more as compared to 1.2 litre engine. Correct me if I am wrong- experts!
amit_purohit20 is offline   Reply With Quote
Old 1st September 2011, 22:32   #5
BHPian
 
lambdaplus's Avatar
 
Join Date: Aug 2010
Location: Bangalore
Posts: 38
Thanked: 38 Times
Default Re: Questions about fuel consumption at engine level

Quote:
I believe that the whole point of the ECU is to achieve complete fuel consumption. That means that the petrol to air mixture needs to be just right. Now that would have been simple if it weren't for varying fuel quality and/or composition.
That's a Bingo. The main aim of Electronic control is to reduce emissions and make the world a greener place!

Quote:
Not exactly. O2 sensor is basically for feedback. The amount of injection depends on air pressure and temperature in the intake manifold.
As Oxyzen pointed out, the O2 sensor(s) are placed in the exhaust manifold and before &\ after the catalyst. Its only for feedback if fuel injected is completely burnt or not. Fuel injection depends on the amount of air that has come in (For petrol cars) through the air filter.

Quote:
1. Does compression ratio have any effect on this?
Yes! Higher the compression ratio, more efficiency & more power can be produced. However, not all engines can sustain high compression ratios as high amounts of heat is produced and also petrol can knock. Thus, the CR is limited to around 10-12.
Knocking also happens depending on fuel composition. Thus, If a car can sense the knocking characteristics of the fuel, it can take some corrective action. Indian cars like Ritz, Swift, Fiesta etc have knock sensors. You can find out if you get slightly better mileage while using higher quality petrol. This is among other reasons, due to the knock sensor.

Quote:
I'll just post a series of theoretical scenarios.
Before the scenarios, lets get a few things clear.
- more throttle opening, better efficiency since the engine doesn't have to do work drawing air from a not-so-open throttle. Remember, a throttle is called so because it restricts air flow into the cylinders! Imagine pulling out a syringe plunger with the nozzle closed.
- higher revs mean higher rate of fuel burning. Hence more fuel is consumed. However, if you go sufficiently fast at high rpms, you reach your destination faster thereby getting a good kmpl reading

Quote:
1. 50% throttle 2500rpm 80kph 4th gear OR
50% throttle 2000rpm 80kph 5th gear
Since throttle opening is same, we can ignore that. And, since rpm is lower in 5th gear, the second option is better in FE.

Quote:
2. 50% throttle 2500rpm 80kph 4th gear OR
40% throttle 2000rpm 80kph 5th gear
Since both throttle is opened lesser in second case, it depends on engine size. Big engines (1.6 and above) have fairly large intake manifolds. Hence, even at 40% throttle, the resistance to piston is not that high. However in smaller engines (800cc, 1.2L etc.,) 40% throttle could be some work for the pistons.
Another thing that we should consider is the gear box design. If the 5th gear has overdrive, then there will be better FE.
Anyway, I think, the second option still has better FE here.

Quote:
3. 40% throttle 2500rpm 80kph 4th gear OR
50% throttle 2000rpm 80kph 5th gear
The first option is kind of better since, the throttle is more open in second case which means, the engine is more loaded. Maybe you are going up a gradient or carrying more weight and that's why the rpm has gone down. Maybe you will have to shift down to keep the vehicle moving at the same speed any moment.

Quote:
Now, we know that a turbo works by drawing in more air into the engine by using energy from exhaust gases.
But if more air is being drawn into the engine, won't the ECU be forced to compensate by pumping in more fuel, thus increasing fuel consumption?
Well, in case of charging, you are partially correct that forcing more air leads to more fuel injection. However, there are some advantages. By pressurizing the intake manifold, throttle losses can be reduced. The exhaust back pressure can be reduced. More torque can be produced, especially at lower rpms as well. Also, the volumetric efficiency of the engine would be higher. Thus, we can't directly compare a NA with TC engine. Its based on how its used.

Quote:
Car 1 has a 3.0liter V6 running at 30% efficiency (rpm doesn't matter)
Car 2 has a 1.2liter I4 running at 30% efficiency (rpm doesn't matter)
A 3L engine running at 30% efficiency by rule of thumb consumes more fuel than a 1.2L engine at same efficiency! However, if you consider a constant torque or power output, then, depending on the operating point, the FE will vary. Here, rpm also would come into play.

If you remember, in Top Gear once they had tested a Prius (hybrid) and a BMW M3. The Prius was driven at its limit and the M3 was following at same speed. Here, the Prius, although a smaller engine consumes more fuel since its run at almost max rpm. i.e., it is producing its max power. However, for an M3, the max power of the Prius is probably its cruising power and hence it consumes much lower fuel in spite of being a bigger engine. Hence, the operating point is vital. An engine has its most fuel efficient operating point and if driven there, it will give best FE. However, for best torque or power, you need to be at a different operating point which will invariably consume more fuel.

Quote:
And what exactly are the brake specific fuel consumption graphs? Can they be applied to real life?
Plainly said, BSFC is fuel consumed to produce a certain power.
For any engine, power produced depends on its rpm. Thus, you can plot a graph of power vs rpm and you can measure the fuel consumption (generally in grams per second) at each power or rpm interval and mark it on that graph. Thus, irrespective of the engine size and other dynamics, you can get a graph of power vs rpm and know how exactly one should use the engine to get best power or FE. It is quite practical for real life, but, no one gives these details.
lambdaplus is offline   Reply With Quote
Old 30th July 2012, 19:29   #6
BHPian
 
Join Date: Jan 2012
Location: Bangalore,Coorg
Posts: 720
Thanked: 303 Times
Exclamation Fuel Efficiency determined by rpm, gear or both?

Is an engine running at 2,000 rpm going to give the same mileage in 1st gear as it would do in 5th gear? I ask because you always see recommendations of the best speed to drive a vehicle at for optimum mileage, but if the mileage were identical irrespective of gear, then the better recommendation would be to keep for the rpm instead of speed. I do understand that speed is something easily understood by the masses, which is why it is used, but are they strictly accurate?

I await input from all you gurus out there.
pganapathy is offline   Reply With Quote
Old 30th July 2012, 20:02   #7
Senior - BHPian
 
R2D2's Avatar
 
Join Date: Oct 2008
Location: Punya Nagari
Posts: 1,167
Thanked: 230 Times
Default re: Fuel Efficiency determined by rpm, gear or both?

Quote:
Originally Posted by pganapathy View Post
Is an engine running at 2,000 rpm going to give the same mileage in 1st gear as it would do in 5th gear?
It cannot. Mileage is basically the distance travelled by the amount of fuel consumed expressed in KM/L or L/100 KM. A car will travel a shorter distance in low gears (1st/2nd) than in higher gears (4th/5th/6th)

So, If engine RPM is held constant @ 2K RPM as you point out and assuming fuel consumed by the engine at that RPM is also constant, the gearbox will increase road speed i.e. (wheel RPM) as you shift upwards from 1st thru 5th/6th by changing the gear ratios. A car might do 20 kmph in 1st @ 2K RPM and 100 kmph in 5th at 2K RPM.

Keeping the same ratio in the example above and a constant unit of time say1 hour, if in 1st gear at 2K RPM you travel 1 KM in that hour, in 5th gear you may travel 5 KM in that hour.

So for the constant amount of fuel consumed (assumed above), you travel a longer distance in 5th/6th (5 km) than in 1st (1 KM) which leads to better fuel consumption KM/L values.

PS - every engine has an optimum engine speed AKA RPM where power, torque, mileage and wear and tear are balanced. That's why they recommend keeping the RPM in a particular range and it is almost always referred to in top gear in 4/5/6th gear as the case may be

Last edited by R2D2 : 30th July 2012 at 20:05.
R2D2 is offline   (5) Thanks Reply With Quote
Old 30th July 2012, 20:05   #8
BHPian
 
Join Date: Mar 2008
Location: Chennai , Tamil Nadu
Posts: 273
Thanked: 45 Times
Default re: Fuel Efficiency determined by rpm, gear or both?

Quote:
Originally Posted by R2D2 View Post
So, If engine RPM is held constant @ 2K RPM as you point out and assuming fuel consumed by the engine at that RPM is also constant, the gearbox will increase road speed i.e. (wheel RPM)
Keeping the same ratio in the example above and a constant unit of time say1 hour, if in 1st gear at 2K RPM you travel 1 KM in that hour, in 5th gear you may travel 5 KM in that hour.
Spot on! Well explained R2D2

This is exactly right. At first gear even though you are at 2k rpm you don't cover much distance. Hence the km per litre you cover is lesser than compared to higher gears say 5th where in you are 80kmph at 2k rpm and covering a much greater distance per second.
kaush666 is offline   (2) Thanks Reply With Quote
Old 30th July 2012, 20:07   #9
BHPian
 
neel385's Avatar
 
Join Date: Nov 2007
Location: Kharagpur
Posts: 473
Thanked: 163 Times
Default re: Fuel Efficiency determined by rpm, gear or both?

Totally agree with R2D2. You might want to find out what is the most efficient RPM for the engine you are driving (you can do this by co-relating the most economical speeds generally given in the manuals with the RPM)and try to keep the revs as close to the ideal as possible in all the gears so as to get the best possible mileage.
neel385 is offline   (1) Thanks Reply With Quote
Old 30th July 2012, 20:34   #10
BHPian
 
Join Date: Jan 2012
Location: Bangalore,Coorg
Posts: 720
Thanked: 303 Times
Default re: Fuel Efficiency determined by rpm, gear or both?

Thanks for that explanation r2d2. Never thought of it in those terms. Actually makes sense. So best mileage would be at lowest revs to keep vehicle running smoothly in the highest gear.

Therefore the answer to my question is a combination of engine revs and gear.
pganapathy is offline   Reply With Quote
Old 30th July 2012, 20:54   #11
Senior - BHPian
 
R2D2's Avatar
 
Join Date: Oct 2008
Location: Punya Nagari
Posts: 1,167
Thanked: 230 Times
Default re: Fuel Efficiency determined by rpm, gear or both?

Quote:
Originally Posted by pganapathy View Post
Thanks for that explanation r2d2. Never thought of it in those terms. Actually makes sense. So best mileage would be at lowest revs to keep vehicle running smoothly in the highest gear.
Therefore the answer to my question is a combination of engine revs and gear.
I think what I need to clarify is that engine speed and road speed are intertwined. If a car mfr specifies an optimum speed it is generally in top gear. Some also specify what is the ideal RPM and max speed in each gear. NEVER redline the car for extended periods. It can cause serious engine damage.

Try and stick to their recommendations for best fuel economy. For e.g. if your car mfr says 70 kmph is optimum speed for enhanced mileage it is always in top gear.

Modern petrol engines spend most of their lives operating between 1500-3500 RPM whether it be in city or out on the highway. Does not apply to speed maniacs please. Drive safe!

Cheers!

Last edited by R2D2 : 30th July 2012 at 20:56.
R2D2 is offline   (1) Thanks Reply With Quote
Old 30th July 2012, 23:51   #12
BHPian
 
paras211's Avatar
 
Join Date: Nov 2006
Location: mumbai
Posts: 582
Thanked: 20 Times
Default

Quote:
Originally Posted by pganapathy
Thanks for that explanation r2d2. Never thought of it in those terms. Actually makes sense. So best mileage would be at lowest revs to keep vehicle running smoothly in the highest gear.

Therefore the answer to my question is a combination of engine revs and gear.
Not true for turbo charged vehicles, they will probably run smooth but won't be efficient unless you get to turbo spooling.
Example the swift ddis will be fine at 1500 rpm but will be more efficient at 2200-2500rpm in top gear.
paras211 is offline   (1) Thanks Reply With Quote
Old 31st July 2012, 00:12   #13
Distinguished - BHPian
 
Join Date: Oct 2011
Location: Hyderabad
Posts: 4,222
Thanked: 2,926 Times
Default re: Fuel Efficiency determined by rpm, gear or both?

TCIC engines would be efficient just when the turbo starts spooling that is near about 2000rpm. Once the turbo is ON then fuel burnt also would be high so decreasing the mileage a bit. Below 1500 rpm the engine would lug which again is bad as redlining the engine.

Diesels are always efficient near the Max torque RPM.
a4anurag is offline   Reply With Quote
Old 31st July 2012, 00:43   #14
Senior - BHPian
 
Join Date: Jul 2006
Location: Madras/Py
Posts: 7,550
Thanked: 403 Times
Default re: Fuel Efficiency determined by rpm, gear or both?

RPM is what defines, but it also matters at what gears, isn't this a basic fact of science ? I do not understand fitting speed governors on city buses, governors should be based on rev limits and not on speed limits to avoid rash driving.
esteem_lover is offline   Reply With Quote
Old 31st July 2012, 01:46   #15
Senior - BHPian
 
Join Date: Dec 2006
Location: Near NH 209
Posts: 1,243
Thanked: 343 Times
Default re: Fuel Efficiency determined by rpm, gear or both?

Quote:
Originally Posted by esteem_lover View Post
I do not understand fitting speed governors on city buses, governors should be based on rev limits and not on speed limits to avoid rash driving.
John sir, don't they need to go to lower gears at higher rpms to climb an incline?
ramzsys is online now   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Diesel engine stalls when the fuel level is low Samartha Technical Stuff 23 22nd August 2014 12:02
idling fuel consumption and pollution passionpurple Technical Stuff 30 28th September 2011 14:38
AC Fuel consumption... scorpio118 Technical Stuff 7 6th May 2007 21:59
Fuel Consumption while standing still amitverma Technical Stuff 26 29th June 2005 10:26


All times are GMT +5.5. The time now is 20:57.

Copyright ©2000 - 2014, Team-BHP.com
Proudly powered by E2E Networks