Mechanical Engineering - Thermodynamics - Discussion

The correct answer is: cut-off is decreased.

The Diesel cycle and Otto cycle are both idealized thermodynamic cycles used to model internal combustion engines. The main difference between them is the way heat is added to the system.

As the cut-off ratio (the ratio of the volume at the end of combustion to the volume at the beginning of combustion) in a Diesel cycle decreases, the cycle approaches the Otto cycle. This is because a smaller cut-off ratio means that the combustion process is more similar to the constant-volume heat addition process in the Otto cycle.

In contrast, a large cut-off ratio means that the combustion process is more similar to the constant-pressure heat addition process in the Diesel cycle. A cut-off ratio of zero would mean no heat addition and a constant cut-off ratio would not change the fundamental characteristics of the cycle.

The compression ratio is defined as the total cylinder volume when the piston is at the bottom dead center to the clearance volume.

Cutoff ratio-Say Fuel is injected into combustion chamber where only air compressed and is at high temperature. Fuel is injected for a duration of time, say T. The piston would not have reached the bottom dead centre in time T. The fuel is cut off when the volume is, say V2. The clearance volume is V1. The ratio V2/V1 is cut off ratio.

As far as the inefficiencies go the formula for for both diesel and Otto cycle have compression ratio in their formula. I don't understand how cut off ratio comes into play. If any one can find a suitable answer please post it and till that time even I am looking into this question.

The cut off ratio is defined on basis of heat addition process. So, say if its isochoric as in petrol cut off ratio is one. And for diesel it is Isobaric process giving ratio greater than one, in my opinion when it reaches unity the efficiency reach to that of petrol Otto cycle.

Mind it I say reaches, not 'is equal to', as it would be like Lenoir cycle with 3 processes,not exactly like Lenoir.

It is safer to say the diesel cycle efficiency will reach that of petrol cycle (or a combined cycle to be precise) as cutoff ratio approaches unity

Correct answer is b.

Becomes its clearly mentioned APPROACHES not When EQUAL. We can't say when cutt of ratio is zero its totally useles the question is telling you when n of diesel Engine APPROACHES. This means you have to decrease cutt of i.e. Rc !

Then it vill approach towards otto, it does not tell you when efficency of diesel eng is equal to that of otto. Even if it had mebtioned we still can't put RC (CUTT OFF) equal to zero.

Hence answer is B.

No,

Here the term used "approached". Not used equal to. Understand the key.

The Answer must be cut off ratio is 0.

Many guys can think, it can be 1 then also it can be right. But, when cut off ratio is 1, the cycle will be comprised of 3 processes, which is impossible. Along with that, it will reach in the limit of 0/0 form.

Here, the Correct answer must be: When cut-off ratio is 0, the diesel cycle approached to Otto cycle.

If cut off ratio tends to unity then the efficiency of diesel cycle approaches to otto cycle.

Diesel cycle efficiency = 1 - ((1/Y*r^ (y-1) * lim ((Ρ^y -1) / (Ρ-1))).

Ρ-> 1.

As a limit term become 0/0, so applying L hospital's law. We get any term which cancels out why in the first term. So diesel cycle efficiency exactly becomes equal to otto cycle.

Practically I don't whether it is possible or not.

The answer is right.

Because here (Cut-off) is mention so, when cut-off will be zero then the efficiency of diesel cycle approaches to the efficiency of otto.

Where CUT-OFF and CUT-OFF RATIO both are different, If cut-off ratio is unity then the efficiency of the diesel cycle will approach to efficiency of otto cycle.

By L-HOSPITAL rule, you can prove, but for cut-off ratio.

If cut off ratio tend to 1, then find the limiting value of diesel cycle efficiency as cut off ratio tend to 1, it will be equal to otto cycle efficiency. Also for those who are considering the cut off ratio zero, right answer, if cutt off ratio is zero means the volume of flue gases after combustion is zero, how can it be possible that flue gases will have zero volume.

The answer is right.

Because here (Cut-off) is mention so, when cut-off will be zero then efficiency of diesel cycle approaches to efficiency of otto.

Mind it, CUT-OFF and CUT-OFF RATIO both are different, If cut-off ratio is unity then efficiency of diesel cycle will approaches to efficiency of otto cycle.

By L-HOSPITAL rule, you can prove, but for cut-off ratio.

Efficiency of diesel cycle is:
1- [(p^(Y)-1)/r^(y-1)Y(p-1)].

When we apply limit with L-hospital rule we get,

Efficiency of otto:
1-[1/r^(y-1)]

Where: r = compression ratio = Volume before compression/Volume after compression.
p = cut-off ratio = volume after injection/volume before injection.
y = Ratio of specific heats or adiabatic index.