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Old 21st February 2006, 11:38   #1
jat
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Default Factors affecting Thermal eff vs FE and diesel vs petrol etc

Many people are confused about the difference between the THERMAL EFFICIENCY of petrol and diesel engines and again with FUEL ECONOMY of cars. Here is the explaination to avoid confusion.
The link below shows a petrol engine curve - ABDE superimposed on a diesel curve - ABCDE (Modern diesel follows a little different curve - part petrol and part diesel curve called DUAL COMBUSTION CYCLE). Please note that these are theoritical curves and actual curves are little different than the ones shown here but almost same within the boundaries defined by these curves and corners won't be so sharpely defined.




While the petrol engine (compression ratio shown here is 9:1) fired by spark follows the pink line - BD (when heat is added), the diesel compression stroke continues (ratio shown as 18:1) till it reaches the point C. From the graph it is clear that in diesel cycle, approx 30% of more output is produced in the same cycle (area left of pink line - BCDB). The heat is added during the period CD and it can be shown by calculation that it is around 13% more than the petrol cycle. So it can be observed that the diesel engine is around 16% more efficient than the petrol engines. And the thermal efficiency is higher due to additional work output available (area left of pink line) because of higher compression ratio. Another factor which affects the thermal efficiency of diesel is the length of line CD (injection begins at C and ends at C -> ratio of vol at D : vol at C is called cutoff ratio), shorter it is more efficient the engine is. And to achieve it higher fuel pressures are required. And enters CRDi of ACCENT or PD of SKODA.


OT - How Pete's tuning box works?
Well IMO, the processor senses the length of CD and then increases the length by continuing the injection beyond the point D by a small amount. Now there is no change in maximum pressure and hence no modifications for mechanical components is required. And as this extra fuel injection is done during flooring the pedal action, and not always, the thermal stresses are kept in check by coolant and lubricating oil. Extra fuel requires more air here. NA engines requires extra means to supply extra air required and hence this device won't be able to work on NA engines or built in supercharger where air flow cannot be adjusted. But the turbos are free running and are generally self balancing, that is increase in fuel will increase turbo rpm and more air will be supplied. Combustion remains good as excess air is always supplied. What about thermal efficiency? During normal running, the length of line CD is decided by outside factors eg wind speed, gradient, gross weight of car etc and will remain same, with or without tuning box. And hence the fuel economy test will return the same readings as these are done at constant speed. So the tuning box turns out to be a smart device, right?



This other link below is for a curve sfc (or efficiency) vs torque for a carburetted engine at a certain rpm, when the AF ratio is varied. Curve for MPFI will be also similar. To understand how the air fuel ratio and also the gear affects the fuel fuel economy while driving, this curve will be helpful.




Let us assume that this curve is for around 1000 rpm that is around idling rpm where the volumetric efficiency of the engine is maximum. When engine is idling, the torque is minimum, fuel injected in air stream is minimum and hence air fuel ratio is maximum (towards the point A). Thermal efficiency is low and sfc is high as cooling affect of air is prominent here and does not allow rapid combustion. When the car is put in first gear, the torque is increased and fuel injected is also increased and hence the air fuel ratio moves towards point B. From the curve, it is evident that the sfc is lowered and therefore fuel economy is increased. Here rate of combustion increases due to A/F ratio moving towards stoichiometric ratio. The more we move to higher gear, the more the operating point moves towards point B. The fuel economy improves as a result till the A/F ratio is around 16. Therefore the highest gear gives the best fuel economy.

When the accelerator is floored suddenly, the engine rpm is still same with air flow being same, but the fuel has increased beyond B and hence the A/F ratio may have reached lower than stoichiometric ratio. The torque produced is now around maximum and acceleration is very high but sfc is also quite high. Thus high rate of acceleration is going to increase the fuel consumption.

Two other curves one for bmp/sfc against rpm and other for vol eff vs rpm has been added to analyse and to give better understanding of above discussion.

RK
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Old 21st February 2006, 12:24   #2
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good write up jat, though it took some time for me understand. why dont you/anyone start a series of threads to throw us light on such technical stuff on a regular basis.

may be we can discuss about it for a week and start with another one...

regards
tifosi.
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Old 21st February 2006, 18:04   #3
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[FONT=Times New Roman][SIZE=3]Hey..[/SIZE][/FONT]
[FONT=Times New Roman][SIZE=3] [/SIZE][/FONT]
[FONT=Times New Roman][SIZE=3]To continue this thread, I’d like to add a few things.[/SIZE][/FONT]
[FONT=Times New Roman][SIZE=3] [/SIZE][/FONT]
[FONT=Times New Roman][SIZE=3]First let me do petrols, because that's what majority of us own.[/SIZE][/FONT]
[FONT=Times New Roman][SIZE=3] [/SIZE][/FONT]
[FONT=Times New Roman][SIZE=3]We all know that to burn anything, we need air. Now there is a specific Air-Fuel ratio that is supposed to bring about optimum combustion. This is called the stoichometric A/F ratio and its value is 14.7 . In petrol engines, since the A/F mixture is ‘lit’ by the spark plug, for best results they should operate at the stoichometric value. This is where the throttle valve comes in. this valve is the one that is activated by the accelerator pedal. When you press the accelerator, the valve opens and air-flow through the engine increases, thus causing the EMS to increase the amount of fuel, to maintain an A/F ratio of 14.7 or near about that. Which is why, OEMs always say that you should drive in fifth (or the highest gear) between speeds of 50-60 kph for optimum fuel economy.[/SIZE][/FONT]
[FONT=Times New Roman][SIZE=3]The feed back on the amount of air coming in to the engine is given by the lambda sensor, [/SIZE][/FONT][SIZE=3][FONT=Times New Roman]which senses the oxygen level in the exhaust and thus adjusts the fuelling. This is also the reason why exhaust temperatures are higher in petrol engines. [/FONT][/SIZE]
[FONT=Times New Roman][SIZE=3] [/SIZE][/FONT]
[FONT=Times New Roman][SIZE=3]Keeping this in mind, if you install anything that INCREASES the air-flow through the engine, such as a larger throttle body, larger header, turbo etc, you can see why the FE would drop. In case of free flow exhausts, as the back pressure on the engine decreases, it allows the engine to produce more power keeping the air flow same (pumping losses decrease). So these may not impact the FE much.[/SIZE][/FONT]
[FONT=Times New Roman][SIZE=3] [/SIZE][/FONT]
[FONT=Times New Roman][SIZE=3]Next. Diesels…[/SIZE][/FONT]
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Old 21st February 2006, 23:09   #4
jat
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Quote:
Originally Posted by tifosikrishna
good write up jat, though it took some time for me understand. why dont you/anyone start a series of threads to throw us light on such technical stuff on a regular basis.

may be we can discuss about it for a week and start with another one...

regards
tifosi.
Thanks.

I do agree that this sort of topic is required so that our guys don't have to look for details on other sites. I got the idea from thred by sbasak - Physics behind car's movement.

RK
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