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Old 30th September 2013, 17:39   #1
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Post Do High RPMs result in unburnt fuel?

A thing got me thinking. In a diesel engine(naturally aspirated), when the engine reaches very high RPMs(fully depressed accelerator), obviously the Fuel Injection system injects more fuel into the cylinder. Now, at high RPMs the pistons move up & down faster than they normally would at low RPMs, naturally the intake & exhaust valves would open and close faster too. So definitely the time taken for the induction stroke would be less.

Would this result in a slightly rich mixture inside the cylinder because of the fast induction process? and would this also be the cause of unburnt fuel at high RPMs?
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Old 30th September 2013, 18:38   #2
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Yes, I think so. Though manufacturers thouroughly perform tests and (might) make the ECU settings such that only the amount of fuel which can be burned at that rpm should be injected. But there is a possibility that some fuel stays unburnt.
Frankly speaking, revving a diesel too much, is useless, for example my Beat's engine produces max torque of 150 Nm in the range of 1700 to 2200 rpms. It produces max power at 4000rpm, but at that rpm, the torque being produced is around 70-80 Nm (approx calculation), which is almost almost half the max torque.
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Old 30th September 2013, 20:19   #3
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Default Re: Does High RPMs result in unburnt fuel?

Good question and I'd really like some senior members to comment on this one.

My belief and from what I have read, the new generation engines with exhaust gas recirculation (EGR) systems considerably bring down the levels of unburnt fuel. The pressure from fuel pump is also regulated based on engine needs. Today's engines are more or less drive by wire with the engine computer making the decisions for the best air fuel mix.

For a naturally aspirated engine its is a different case. These engines depend on atmospheric pressure to get their air fuel mixture. The vacuum created by the piston which sucks in air and fuel. The fuel here is regulated mechanically by the accelerator pedal. A push of the pedal means more diesel input which might not always translate to correct mix for optimum combustion.

Last edited by HillMan : 30th September 2013 at 20:21.
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Old 1st October 2013, 10:01   #4
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Default Re: Do High RPMs result in unburnt fuel?

You are right about the time for suction stroke being low at high RPM, I believe that the speed of suction would also be more at higher RPMs hence time for creating vacuum is less.

I agree that at full throttle the engine might not deliver the highest power rating, but would one be able to push the vehicle maximum RPM? I mean, after a certain point of time, no matter how hard one accelerates, the RPM does not go up. (This has a lot to do with drive-train).
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Old 1st October 2013, 12:05   #5
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Default Re: Do High RPMs result in unburnt fuel?

Quote:
Originally Posted by katchkamalesh View Post
I agree that at full throttle the engine might not deliver the highest power rating, but would one be able to push the vehicle maximum RPM?
The new generation engines have RPM limiters to avoid harming the various components. Translates to lesser warranty claims and longer engine life.

On the older generation engines the driver had to feel the engines to take them to the limits. But you have made a good point and logical as well.

At higher RPM, the air flow into the piston is limited from various laws of physics. Fuel pressure was maintained by a mechanical pump run by the engine rotation. This will cause lesser air for compression and more fuel because you would be flooring the paddles. Naturally torque goes down exponentially as the speed increases.

So I guess, yes, there will be a lot of unburnt fuel as the RPM increases in a naturally aspirated engines.
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Old 1st October 2013, 15:10   #6
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Default Re: Do High RPMs result in unburnt fuel?

Quote:
Originally Posted by sr_1445 View Post
... In a diesel engine(naturally aspirated), when the engine reaches very high RPMs(fully depressed accelerator), obviously the Fuel Injection system injects more fuel into the cylinder. ...

Would this result in a slightly rich mixture inside the cylinder because of the fast induction process? and would this also be the cause of unburnt fuel at high RPMs?
No. By design, the mixture is slightly lean under almost all circumstances.

The key dependency here is injection control. Unlike carburetted petrols, the amount of fuel in the cylinder is not a 'happening', i.e. fuel is not carried into the cylinder with air (that is what 'induction process' would imply). In production engines, quantity of fuel injected at different RPMs is calibrated on a test bench (a couple of engines in a lot, not all).

* If the injection control is mechanical (in-line or distributor pumps without electronic control), one can consider gap tolerances and fuel pump piston lag (at 'full speed) to figure out whether there is excess fuel being injected

* If the injection control is electronic (control of time the injector is kept open), the only suspect is the spring that holds the injector shut - the control program doesn't deliberately inject more fuel unless the situation demands it (e.g. kick-down - an indication by driver of higher power demand for a short time)

Anyhow, excess fuel will always manifest itself in higher partially burnt components in exhaust, typically black or dark grey smoke. If the vehicle is not accompanied by a billowing cloud of black smoke, don't worry - injected quantity is just fine!
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Old 1st October 2013, 15:26   #7
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Default Re: Do High RPMs result in unburnt fuel?

+1 to DerAlte. Any incomplete combustion would result in visible smoke. Hence, if you can see smoke, then the fuel is not being combusted fully.
More than worrying at the situation where engine is running at a high RPM, I think much more fuel is wasted when one is lugging at a higher gear with low speed and pressing the accelerator pedal and waiting for the engine to build up substantial revs for that gear. A lot of unburnt fuel is wasted this way.
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Old 1st October 2013, 16:52   #8
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Default Re: Do High RPMs result in unburnt fuel?

In my Punto 75HP, when I drive at RPM between 1200 and 2000, I get a very high efficiency - 22-25kmpl in normal driving conditions. However, if I keep the RPM range between 2500-4000 for a long time, the efficiency drops drastically - to the range of 15-16kmpl, in the same conditions.

Other than unburnt fuel, what else could be contributors to this drop in efficiency?
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Old 1st October 2013, 17:09   #9
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Default Re: Do High RPMs result in unburnt fuel?

Quote:
Originally Posted by devansn View Post
Other than unburnt fuel, what else could be contributors to this drop in efficiency?
In your case, it is clearly not the unburnt fuel that is causing the mileage drop. It is more to do with wind resistance. Modern engines are very efficient and have many sensors and a computer to control the engine parameters.

On my Duster, I noticed a drop in 0.6kmpl in FE when the windows are rolled down on highway traveling at 80kmph.
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Old 1st October 2013, 17:20   #10
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Default Re: Do High RPMs result in unburnt fuel?

Quote:
Originally Posted by devansn View Post
Other than unburnt fuel, what else could be contributors to this drop in efficiency?
Wastage of fuel due to improper channelization of energy. During such enthusiastic driving, often we tend to pull through each gears by redlining in every gear. Now when the top speed for a gear is achieved, accelerating further only increases the engine RPM and not the vehicle speed which means that the extra RPM is not converted into distance travelled. Hence, fuel gets wasted. So, I would say that improper gear shifts & uselessly redlining is the culprit here.

Though, there are other factors too. When we accelerate faster, more fuel is injected in the cylinders. There is always a lag when engine can actually utilize all of the fuel by reaching that particular RPM range. Again, this results in loss of FE. Hence, one can save a lot of fuel when accelerating sedately & in a linear manner.

Last but not the least, each engine has its own sweet range to operate, mainly depending upon the gear ratios & the size of the engine in terms of cubic capacity. My car does not have an RPM gauge, but I can feel when it is operating in its sweet range, and that is the range which will give me the best FE. For my 1000 CC Zen, it gives great FE when accelerated sedately and finally settling at 60-70KMPH in the 5th gear.

Regards.
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Old 1st October 2013, 17:32   #11
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Default Re: Do High RPMs result in unburnt fuel?

Quote:
Originally Posted by HillMan View Post
In your case, it is clearly not the unburnt fuel that is causing the mileage drop. It is more to do with wind resistance. Modern engines are very efficient and have many sensors and a computer to control the engine parameters.

On my Duster, I noticed a drop in 0.6kmpl in FE when the windows are rolled down on highway traveling at 80kmph.
Thanks for the clarification, HillMan. Wind Resistance causing this much difference (5-6 kmpl) with windows rolled up, actually did not come to my mind at all...


Saket, in my case, I think its the wind resistance only. Because, even when I reach higher speeds, I almost always reach there sedately. So yes, over-revving happens only on the fifth gear. And on various online documents, I see that almost 30% difference can be seen in the efficiency by reducing the speed from 75mph to 55 mph (120 kmph to 85 kmph or so), due to drag.

Last edited by devansn : 1st October 2013 at 17:39. Reason: Added reply for Saket
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Old 2nd October 2013, 00:35   #12
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Default Re: Do High RPMs result in unburnt fuel?

Quote:
Originally Posted by devansn View Post
In my Punto 75HP, when I drive at RPM between 1200 and 2000, I get a very high efficiency - 22-25kmpl in normal driving conditions. However, if I keep the RPM range between 2500-4000 for a long time, the efficiency drops drastically - to the range of 15-16kmpl, in the same conditions.

Other than unburnt fuel, what else could be contributors to this drop in efficiency?
Each engine inherently has its best fuel efficiency in a certain RPM range. A typical BSFC (Brake Specific Fuel Consumption) chart for an engine will look like U shaped.

http://ecomodder.com/wiki/index.php/...on_(BSFC)_Maps

So to produce a similar power output at a higher than optimal RPM, the engine requires more fuel.

As far as unburnt fuel goes, modern ECUs and Electronic Fuel Injection systems minimise that using the help of stored optimal fuel-air mixture tables and inputs from the Mass Air Flow Sensor (MAF Sensor) on the intake side and the Oxygen Sensor (O2 Sensor) on the exhaust side.
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Old 2nd October 2013, 05:41   #13
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Default Re: Do High RPMs result in unburnt fuel?

To overcome this escape of unburnt charge during higher engine RPMs is variable valve timing and variable ignition timing controls are all about. These coupled with variable geometry turbocharger prevents escape of unburnt charge and improves efficiency. Turbochargers using variable turbine geometry, offer an almost constant boost pressure across the widest rev range.

Last edited by rajeev k : 2nd October 2013 at 05:51.
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Old 2nd October 2013, 22:01   #14
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Default Re: Do High RPMs result in unburnt fuel?

Quote:
Originally Posted by sr_1445 View Post
In a diesel engine(naturally aspirated), when the engine reaches very high RPMs(fully depressed accelerator), obviously the Fuel Injection system injects more fuel into the cylinder.
Would this result in a slightly rich mixture inside the cylinder because of the fast induction process? and would this also be the cause of unburnt fuel at high RPMs?
Why do you specify naturally aspirated diesel engines only?

Fully depressed accelerator need not mean max RPM that the engine is capable of. Is your query related to loaded or free-revving condition?

What kind of fuel injection system? Jerk injection systems, or common rail? Is your engine ECM-controlled or not?
Quote:
Originally Posted by DerAlte View Post
No. By design, the mixture is slightly lean under almost all circumstances.

Anyhow, excess fuel will always manifest itself in higher partially burnt components in exhaust, typically black or dark grey smoke. If the vehicle is not accompanied by a billowing cloud of black smoke, don't worry - injected quantity is just fine!
Sir, is smoke the only indicator for unburnt fuel? What happens at engine start? And at high RPM under loaded and unloaded conditions?
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Old 2nd October 2013, 23:27   #15
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Default Re: Do High RPMs result in unburnt fuel?

Quote:
Originally Posted by SS-Traveller View Post
Why do you specify naturally aspirated diesel engines only?

Fully depressed accelerator need not mean max RPM that the engine is capable of. Is your query related to loaded or free-revving condition?

What kind of fuel injection system? Jerk injection systems, or common rail? Is your engine ECM-controlled or not?

Sir, is smoke the only indicator for unburnt fuel? What happens at engine start? And at high RPM under loaded and unloaded conditions?
Some very good questions!

Just my 2cents worth:

The only way you can get unburnt fuel is fuel that is not lit! If it is lit it gets burned, maybe not very well, partly burned. But unburnt fuel is only possible due to the fact that hasn't been lit or ignited! Otherwise it can't be called "unburnt fuel".

It can happen in engines as some fuel might get trapped in flame avoiding zones, such as crevices provided by the piston rings. Also, it depends on the size and shape of the combustion chamber. Not everywhere there will be a correct temperature to help ignite the fuel. Petrol engines are prone to this effect, but so are Diesels.

I'm not sure but I believe if my chemistry is up to speed what gets emitted is formaldehyde and alkenes.

If it's visible smoke (black) it's likely to be partly burned fuel, not unburnt fuel! If it's white it's likely to be, partly, unburnt fuel. Especially on diesels that start from cold you can see this.

Note, that nowhere is a mention of RPM. Correct, unburnt fuel happens across the full RPM range. But its very present during cold starts, especially on diesels.

Jeroen
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