This document compares and contrasts spark ignition (SI) engines and compression ignition (CI) engines. It states that SI engines, typically gasoline engines, ignite air-fuel mixtures using a spark plug, while CI engines, typically diesel engines, ignite fuel injected into compressed hot air without a spark. It provides details on fuels used, combustion processes, compression ratios, and differences in air/fuel intake and combustion between the two engine types.

1. Title:-Difference between SI and CI engine
Mechanical Engineering Department
Submitted By -
Name : Rupesh Kumar
Section : B
Roll No. : 23
Sem : 6th
Faculty Advisor -
Prof. Kishor Rambhad
(Prof. Mech. Engg. Dept.)
Session 2017-18
Subject – Energy Conversion - II
TAE II: Technical Presentation

2. SI Engines
A spark-ignition engine is an internal combustion engine, generally a petrol
engine, where the combustion process of the air-fuel mixture is ignited by a
spark from a spark plug. This is in contrast to compression-ignition engines,
typically diesel engines, where the heat generated from compression
together with the injection of fuel is enough to initiate the combustion
process, without needing any external spark.
Fuels
Spark-ignition engines are commonly referred to as "gasoline engines" in
North America, and "petrol engines" in Britain and the rest of the world.
However, these terms are not preferred, since spark-ignition engines can
(and increasingly are) run on fuels other than petrol/gasoline, such as
autogas (LPG), methanol, ethanol, bioethanol, compressed natural gas
(CNG), hydrogen, and (in drag racing) nitromethane

3. CI Engines
The diesel engine also known as a compression-ignition or CI engine,
named after Rudolf Diesel, is an internal combustion engine in which ignition
of the fuel which is injected into the combustion chamber is caused by the
elevated temperature of the air in the cylinder due to mechanical compression
(adiabatic compression). Diesel engines work by compressing only the air.
This increases the air temperature inside the cylinder to such a high degree that
atomised diesel fuel that is injected into the combustion chamber ignites
spontaneously.
Diesel engines can operate on a variety of different fuels, depending on
configuration, though the eponymous diesel fuel derived from crude oil is
most common. The engines can work with the full spectrum of crude oil
distillates, from natural gas, alcohols, petrol, wood gas to the fuel oils from
diesel oil to residual fuels. Many automotive diesel engines would run on
100% biodiesel without any modifications.

7. CI vs. SI Engines
• SI engines draw fuel and air into the cylinder.
• Fuel must be injected into the cylinder at the desired time of combustion
in CI engines.
• Air intake is throttled to the SI engine ‐‐ no throttling in CI engines.
• Compression ratios must be high enough to cause auto‐ignition in CI
engines (CI:12 to 24), compressed to pressure about 4 Mpa and
temperature about 800 K.
• Upper compression ratio in SI engines is limited by the auto‐ ignition
temperature (SI: 8 to 12).
• Flame front in SI engines smooth and controlled.
• CI combustion is rapid and uncontrolled at the beginning.
• The valve timing in both CI and SI are similar.

8. • Combustion in a CI engine is quite different from that of a SI engine.
While combustion in an SI engine is essentially a flame front moving
through a homogeneous mixture, combustion in a CI engine is an
unsteady process occurring simultaneously in many spots in a very
non-homogeneous mixture controlled by fuel injection.