Because diesel takes a long time to burn and is injected and combusted during the power stroke. The progressive burn releases heat which would try to increase the pressure, but the cylinder is expanding which decreased pressure due to the increase in volume. In the ideal cycle these balance out. In reality they don't.
It's like the ideal Otto cycle that had heat addition at constant volume. This assumed that the combustion event of a fixed homogenous charge of fuel and air takes place in an infinitely small amount of time. Where in reality it doesn't.
The reason, for constant pressure heat addition in a diesel cycle is that, when the fuel is injected into the compressed air inside the cylinder, question of maintaining a constant volume vanishes. We are adding more volume and igniting the mixture as heat addition. Hence, a constant volume is not possible. And secondarily, the fuel ignition process in a diesel engine is auto-ignition and is caused by pressurizing the Fuel and Air after mixing them.my question was why is the heat addition happening only at constant pressure and why cant it be done at constant volume just like in otto cycle! what is the difference in otto cycle and diesel cycle that makes the heat addition process different ...i was asked to derive this ans in an interview ,hope there is better way to approach for this answer...
What is the difference in otto cycle and diesel cycle that makes the heat addition process different? Let see what the inventor himself (Rudolf Diesel) claim:my question was why is the heat addition happening only at constant pressure and why cant it be done at constant volume just like in otto cycle! what is the difference in otto cycle and diesel cycle that makes the heat addition process different ...i was asked to derive this ans in an interview ,hope there is better way to approach for this answer...
Why cant it be done at constant volume just like in otto cycle? The question should be: What motivated Rudolf Diesel to invent an engine that doesn't have a constant-volume combustion? Again, according to the inventor himself:' WhatI claim as new, and desire to secure by Letters Patent, is--
1. The herein described process for converting the heat energy of fuel into work, consisting in first compressing air, or a mixture of air and neutral gas or vapor, to a degree producing a temperature above the igniting point of the fuel to be consumed, then gradually introducing the fuel for combustion into the compressed air while expanding against a resistance sufliciently to prevent an essential increase of temperature and pressure, then discontinuing the supply of fuel and further expanding without transfer of heat.
2. In an internal combustion engine, the combination with the cylinder and piston, of a valved suction inlet for air or a mixture of air and neutral gas, a valved fuel feed constructed to gradually discharge the fuel into the cylinder, and means in operative connection with the feed valve for opening the same at the commencement of the working stroke of the piston and for closing the same at a predetermined part of the stroke, substantially as described.
3. In an internal character specified, the combination of a combustion cylinder provided with means for gradually introducing fuel therein up to the point of cut-off, a compressor for air, a reservoir connected with the latter and with the cylinder, and an expansion chamber for the exhaust gases, substantially as described.
lleretofore the combustion of the gaseous mixture has been left entirely to itself immediately after ignition, no attempt having been made to regulate or control the pressure and temperature during the combustion with reference to the existing volume of the body of air. From this condition of matters resultthe following disadvantages: First, the temperature produced by the combustion is so high that it is impossible to obtain a mean temperature which will permit lubrication and the maintenance of the parts inproper condition for practical working without the presence of arrangements for cooling the cylinders; second, the products of combustion are insufficiently cooled by expansion and escape while in a hot condition, with the consequent loss of heat and energy. Particular types of the above-mentioned class of engines also possess the same defects.