 Myself, P. V. Ekhande working as assistant professor in Department of Mechanical Engineering at Valkan Institute of Technology, Solapur. Today we will see the comparison between the spark ignition and compression ignition engines that is SI engine and CI engine. At the end of this session, students will be able to explain the difference between SI and CI internal combustion engines. Now, we have already seen regarding the working of diesel engine, petrol engine or the according to the strokes that is two-stroke engine, four-stroke engine or the working of spark ignition and compression ignition that also we have seen. Now, here based on this knowledge, we will see the comparison points between spark ignition and compression ignition engine. Now, the first point of the comparison is in SI engines, we have, there is a provision of spark plug to ignite the air fuel mixture by electric spark and in CI engine, there is a fuel injector to inject fuel and the mixture is ignited by high compression and high temperature developed in the cylinder. So, the first point of the comparison is in case of the spark ignition, there is a provision of the spark plug for igniting the fuel air mixture that is petrol air mixture whereas in case of the CI engine, during the suction stroke only air is taken as a intake which is compressed with help of the large size engine and due to the compression, the pressure increases, temperature also increases and at the end of the compression, there is injection of the fuel that is a diesel with the help of fuel injector that is a sort of nozzle which is spraying the diesel in the very fine particle parts that is called as a say atomized form. So, that is first point of the comparison and that atomized form of the diesel is ignited on its own due to high pressure and high temperature developed in the cylinder. So, this is first point of the comparison. Now, second point of the comparison is related to the fuel, we have seen that in case of the spark ignition, the spark is generated for igniting the petrol air mixture. So, here fuel uses naturally petrol whereas in case of the compression ignition engine, the diesel is ignited at the end of the compression stroke. So, there we are using the diesel as a fuel whereas initially only air is taken as a say charge during the suction stroke. Now, third point is related to the compression ratio. So, compression ratio required for the compression ignition is naturally high and that is required to develop the high pressure and high temperature in the cylinder whereas that is not required for the spark ignition engine. In spark ignition engine, the spark is ignited for the combustion of the fuel. So, we require the lesser compression ratio, so that is up to 9. So, we can say that from 5 to 9 that will be the compression ratio in case of the SI engine whereas it is from around 14 to 20 in case of the compression ignition engine. From this you think suppose we have been brought in front of the engine and the engine is kept in front of you and you have been asked to recognize whether it is the spark ignition or the compression ignition. How you are going to recognize that? Yes, we can recognize the engines as a spark ignition or the compression ignition with the help of the spark plug provided over there. The spark ignition engine is provided with the spark plug whereas in case of the compression ignition it is not provided with the spark plug as the fuel will get ignited on its own because of the fuel injection in the atomized form at the end of the compression or CI engine. Now, the next point or the fourth point of the comparison is in case of the spark ignition engine the pressure and temperature is less as compared to the compression engine. Just now we have seen regarding the compression ratio and due to the lesser compression ratio in case of the SI engine the pressure and temperature develop is naturally less whereas in case of the compression ignition engine the pressure and temperature develop is higher and that is because of the higher compression ratio. So, here the pressure temperature is high and that is why the engine size or the say the more strong engine is required in case of the compression ignition. So, that is the comparison related to the pressure and temperature develop in CI and SI engine. Now, based on that we can say that the construction is light in case of the spark ignition this is one more point of the comparison. So, the construction is light in case of the SI engine spark ignition and the construction is heavy in case of the CI engine or compression engine. Then next point of the comparison is related to the speed. The SI engines operates at a high speed and compression ignition engines operates at a low speed. So, here the speed will be higher because of say generation of the spark and here the CI engines operates at low speed. So, that is the point of the comparison based on the speed and next point is the starting related to the engine starting. In SI engine we are providing the spark for igniting the fuel. So, naturally the starting is easy because once you have the say the starting of the engine in one or two moment of the piston itself the starting will be there as there is a spark provided whereas in case of the CI engine it is somewhat difficult. But that is again we can say that now because of the advanced technology we are not facing that much problem. But as a comparison it is very easy in case of the spark it is not that much. Now, the next point of the comparison that is related to the noise produce. Now, in case of the spark ignition engines the noise produce will be lesser and in case of the CI engine the noise produce is more and that is because of the more vibrations and why the vibrations are more in case of the spark compression ignition engine because the heavy engine is required for generating the high pressure high temperature that means the compression ratio is high that leads to the high pressure and high temperature and naturally the working will have some noise as compared to the say SI engine. So, this is less noise means it is almost nil in some vehicles if you stand nearby the vehicle which is having the spark ignition engine you will not feel the working of that engine also you may think that the engine is a stand steel or the vehicle is a stand steel whereas that is not the case for the compression engine if you stand nearby definitely there will be noise. Now, the next point of the comparison that is related to the maintenance cost of the engine or initial cost of the engine it is less in case of the initial in case of the SI engine. So, initial investment initial cost is also less in because the size and design is required will be a lighter one whereas the case of the CI is different in case of the compression ignition engine the initial cost itself will increase because of the heavy engine heavy design and the maintenance cost is also more this is because of the more moving parts more bulky parts which are provided over there. So, this is the comparison point related to maintenance and the initial cost of the engine and the next point of the comparison is efficiency. So, spark ignition is having the lesser efficiency because the power generated will be more in case of that but the efficiency will decrease in case of the spark ignition whereas the efficiency is higher in case of the compression ignition. So, if you compare the average or the mileage for the same size of the engine suppose 1200 cc engine which is provided with the spark ignition as compared to the same size of the engine of the compression ignition. So, we have the higher efficiency for the compression ignition whereas it is lesser in case of the spark ignition these are the reference books. Thank you.