I need an explanation for this if I have engines that have the same compression ratio.
Dear Mr. Kamal,
Combustion in a constant volume produces higher reaction temperature as compared to the constant pressure combustion.
For liquid fuel combustion, the high temperature helps fuel spray and atomization process, thus improves air-fuel mixing and combustion efficiency.
It also helps to maintain high activation energy for the oxidation to proceed, or in other words, for the flame to propagate.
In contrast, constant pressure combustion has increasing volume which may not favor the flame propagation, especially near the end of the oxidation process.
Regards,
Mas Fawzi
In my opinion, the above answers are sufficient, I just need to add a piece of information regarding Carnot Principle for energy conversion. The thermal efficiency will be increased if either the lower reservoir temperature is reduced and/or the higher temperature reservoir is increased. As the constant volume combustion process provides higher adiabatic flame temperature, then the corresponding higher temperature reservoir will be increased over that provided by a constant pressure combustion process. This result is confirmed by the analysis of air standard cycles (Otto versus Diesel cycles) for the same compression ratio.
The basic contributors for this high efficiency is the lower time available for heat transfer out from combustion charge to engine cylinder head and combustion chamber walls which in turns leads to higher temperatures as mentioned by others
All answers above are sufficient.Main factor responsible is the time avaiable in contant volume heat addition process is less as compared to constant pressure process.Due to rapid rise in temperature and pressure flame propogation is fast as compared to constant volume process.This is the reason of constant volume combustion give an higher indicated fuel conversion efficiency than constant pressure combustion.
It is quite simple, look at your ideal air cycles. Otto cycle vs. Diesel cycle in your basiic thermodynamics course. You get work out of the heat added by expanding it (dW=pdV). When you add heat at constant volume, the pressure is allowed to rise, and all of the energy added can be expanded through the whole compression (you may call it expansion) ratio. At constant pressure, you must expand as the heat is added to keep the pressure from rising. Thus the last heat added is subjected to a smaller expansion ratio, producing less work, and giving a lower efficiency. In the extreme case, if you added enough heat at constant pressure to reach the initial, pre-compression volume, then the last bit of heat added would contribute nothing to the work or efficiency. At the other extreme, for an infinitely small heat addition, the efficiency of the two processes would be the same, though work basically zero in the constant pressure case. All of this is independent of time or heat transfer or evaporation processes and such.
I have designed a constant volume Otto cycle engine, with desmodromic intake and exhaust. It has approx 12% higher efficiency, and much more powerful, being able to rev to over 15,000 rpm. Limit to revs is mechanical failure, not breathing. I am currently testing a prototype. Anyone interested should email me
[email protected]. Ps I originally set out to make Desmo engine, constant volume stuff was something that i noticed after building it, how bizarre!
The reasons for constant volume combustion (eg. petrol) gives more indicated fuel conversion efficiency than constant pressure combustion is: Petrol is a highly volatile fuel, it needs only just a spark to ignite, petrol and air forms a homogeneous mixture, it does not need higher compression ratios to ignite itself and petrol easily burns and gets converted to give power. Whereas, constant pressure combustion (eg. Diesel), is not that volatile it needs to have high compression ratios to ignite, it forms a heterogeneous mixture where the fuel should be added in form of atomization i.e. split into small droplets. It suffers to ignite fully and spontaneous ignition take place at several points in combustion chamber and this needs more air in it i.e. oxygen for complete combustion.
Excelent answers I would just add something that I feel is missing to have the complete picture. Many people indicated that combustion at constant volume produces higher flame temperature than constant pressure combustion. That's just a consequenque of 1st law of TD. At constant pressure, part of the energy is "lost" into pdV work, volume is increasing (remember that at constant pressure you have: h_in=h_out). At constant volume energy goes straight into heating. The rest on how this is achieved has been explained very well before. Hope this helps!
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