ENGINE CLASSIFICATION

Engine is something which help us Humans give power which can be used to perform any particular task.  Imagine we cranking an alternator continously to get electricity!!! Instead we the humans made what is a diesel engine which will do this work for us.  I would say that an engine is something which transforms one form of energy to another and as a result make our life simpler and easier.

An Engine can be:

HEAT ENGINE - Engines which converts heat energy into mechanical or electrical energy.  Heat engines are usually Prime movers.

EXTERNAL COMBUSTION ENGINE (EC ENGINE) - Engines in which the combustion of fuel which is directly or in-directly responsible for driving the engine, takes place outside the engine.  Perfect example of this type engine would be steam turbines, where fuel is burnt in a boiler and the steam produced from the boiler is used for driving the Turbine.

INTERNAL COMBUSTION ENGINE (IC ENGINE)  -Engines in which the combustion of fuel takes place within the engine, are called Internal Combustion Engines.  Simplicity in engine design, operational costs and fuel economy is what makes these engines more popular and efficient than EC Engines.

We are more interested in Internal combustion engines as on board motor ships we have this type of engine installed.

Internal Combustion Engines are Classified on the basis of:

1. IGNITION SYSTEM - Ignition system can be of two types i.e. COMPRESSION IGNITION ENGINES and SPRAK IGNITION ENGINES. 

• COMPRESSION IGNITION ENGINES (CI ENGINES)- In these types of engines, the heat which is produced due to the compression within a cylinder is so high that it is sufficient enough to cause combustion and as a result there is no other means as such provided to cause the ignition of fuel within the cylinder.  A perfect example of CI Engines is a Diesel Engine.  Figure below illustrates the working cycle of a Diesel engine.  You can also click on this link to get the animated view of the diesel engine.  DESEL ENGINE ANIMATION.  These types of engines are based on DIESEL CYLCLE.

• SPARK IGNITION ENGINES (SI ENGINES) - In these type of engines, it is the Spark Plug, which produces a spark, causes the ignition of fuel within the cylinder.  This type of engine is found in our Cars and Bikes which we ride in our day to day routine.  Figure below illustrates the working of a Spark Ignition Engine.

Both SI engines and CI engines are internal combustion engines and both make use of liquid fuels.  There are a number of differences between the two.
Table below shows major differences between SI and CI engines.

p.s. Compression Ratio is defined as the ratio of maximum volume of cylinder i.e with piston at BDC (bottom dead centre) to volume of the cylinder with piston at maximum compression i.e with piston at TDC (top dead centre).  Watch this video to understand Compression Ratio.

2. Internal combustion engines can further be classified on the basis of OPERATING CYCLES

 

• OTTO CYCLE which is also known as CONSTANT VOLUME COMBUSTION CYCLE- It is the standard cycle which is employed in Petrol engines i.e. our car engines.
• DIESEL CYCLE which is also known as CONSTANT PRESSURE COMBUSTION CYCLE - it is a standard cycle which is employed in slow speed Diesel engines.
• DUAL COMBUSTION CYCLE which is also known as CONSTANT PRESSURE AND CONSTANT VOLUME COMBUSTION CYCLE - It is a combination of both otto cycle and diesel cycle where heat is partly added at constant volume and partly at constant pressure.  This type of cycle is employed in Medium and High speed diesel engines.

3.  Internal combustion engines can also be classified on the basis of STROKES or CYCLES
Cycles in a engine means the following events:

• Filling the Engine Cylinder with Fresh Air
• Compressing the air so much that the fuel vapors which are coming in contact with this compressed air which is now at an elevated temperature due to compression ignites
• Combustion of fuel
• Expansion of hot gases
• Exhaust of these gases after moving the piston

In single word, a cycle comprises  of - INTAKE - COMPRESSION - POWER - EXPANSION - EXHAUST.

Depending on many strokes of a piston are required in completing this cycle, the engines are further divided into 2 classes

• FOUR STROKE ENGINES -An engine which requires 4 strokes of piston i.e. 2 times up and 2 times down to complete one cycle, is a Four Stroke Engine.
• TWO STROKE ENGINE - An engine which requires 2 strokes of piston i.e. 1 time up and 1 time down to complete one cycle, is a Two Stroke Engine.

4. Internal combustion engines are also classified on the basis of PISTON ACTION

• SINGLE ACTING ENGINE - Engines of Single Acting type have one One Piston per cylinder, with the pressure of the combustion gases acting only on the surface of the piston.  Single acting Engines are widely used in Internal combustion engines as well as in many external combustion engines.  This is what we have on Board our ships.

Figure shows a typical Single acting Engine

•   DOUBLE ACTING ENGINE - In this type of engine both the ends of the cylinder and both faces of the piston are used to develop power i.e. cylinder develops power in both upward and as well downward stroke.

Figure shows a Typical Double Acting Engine

• OPPOSED PISTON ENGINES - This type of engine comprises of 2 pistons which are traveling in opposite directions.  The combustion space is in the middle of the cylinder and lies between the two pistons.  These engines have 2 crankshafts as well, where the one piston drives one crankshaft and other piston drives the other.  P.S. each piston is single acting.

Figure shows a Typical Opposed Piston Engine

5.  Internal Combustion engines can be classified according to PISTON CONNECTION

• TRUNK PISTON TYPE - In this type of engine piston is connected directly to the upper end of the connecting rod.  A gudgeon pin or a horizontal pin or wrist pin is what connects the two.  This is the type of engine that we have on board our ships for Medium Speed Engines.

Figure shows a typical Trunk Type Piston

• CROSS HEAD TYPE ENGINE - In this type of engine piston is attached to a piston rod whose lower end is connected to a Cross head, which slides up and down in guides.  Cross head is connected to the connected rod.  This type of engines are mainly used in large 2 stroke engines and in double acting engines.

Animation shows a typical Cross head Engine

At this point of our learning it would be nice to Compare the two engines i.e.

TRUNK PISTON V/S CROSS HEAD ENGINES

On board our ships most of the Diesel generator make make use of trunk pistons where as the main propulsion engine is crosshead engine.  

CROSSHEAD ENGINE V/S TRUNK ENGINE

KEY - 1-EXHAUST, 2-SCAVENGE AIR RECEIVER, 3-EXHAUST VALVE , 4-CYLINDER HEAD, 5 -A FRAME, 6-CYLINDER LINER, 7-PISTON, 8-SCAVENGING AIR PORTS, 9-PISTON ROD, 10-CROSSHEAD, 11-COLUMN, 12-CONNECTING ROD, 13-CRANKCASE, 14-BEDPLATE, 15-CRANKSHAFT, 16-INLET VALVE

Most of the medium and small size engines use trunk pistons.  As the piston is being pushed upwards by the crankshaft and the connecting rod during compression, resulting side thrust which is produced which causes the piston to press against the cylinder wall, first on one side, then on the other as it moves down.  Thus side thrust alternates from side to side as piston moves up and down.  At the top stroke, when the gas pressure is greatest, side thrust is negligible (this happens in trunk type engines as there is a small connecting rod angle).  So, most of the wear take place at the middle of the stroke: making piston skirt increases thrust bearing area, and hence reduces wear.  In medium and small size engines, due to lower gas pressure, unit's side pressure is so small that neither piston nor liner wears much.

In crosshead engines, crosshead takes the side thrust, which will be high in large engines.  Cross head engines have the following advantages:

• Easier Lubrication
• Reduced liner wear
• Uniformly distributed clearance around piston
• Simpler piston construction as gudgeon pin and its bearing are note there

On the other hand Cross head engines can have the following disadvantages

• Greater complication in engine construction
• Added weight of crosshead
• Added height due to the addition of another component i.e. Crosshead
• Need careful adjustments

  6.  Internal combustion engines can also be classified according to CYLINDER ARRANGEMENT

• CYLINDER - IN - LINE ARRANGEMENT - This is the simplest and the most common arrangement.  In this type of arrangement all cylinders are vertically in line.

Figure shows 4 cylinders in line, however the number of cylinders can go up to 12 but the most common is 6.

• V-ARRANGEMENT - If an engine has more than 8 cylinders, it becomes difficult to make a sufficiently rigid frame and crankshaft with an inline arrangement.  Also engine becomes quite long and takes up considerable space.  As a result, V-Arrangement is used for engines with more cylinders, (generally 8,12,16) giving about half-length of engine, more rigid and stiff crankshaft, less manufacturing and installing cost.  Angle between 2 cylinders or banks is kept from 30 degree to 120 degrees (most commonly 40 deg, 75 deg)

Figure shows a typical V-Arrangement Engine

• FLAT ARRANGEMENT -  It is a V-Engine, but this type of V engine has an angle between banks increase to 180 degrees.  This type of engine is mainly used in trucks, buses, rail cars etc.

Animation illustrates a typical Flat Engine

• RADIAL ARRANGEMENT - In a radial arrangement engine or Radial Engine all the cylinders are set in a circle and all point towards the centre of the circle.  The connecting rods of all pistons work on a single crankpin, which rotates around the centre of the circle. This type of engine was used in aircraft engines but now turbines are more widely used. 

Figure illustrates a typical Radial Engine

7.  Internal Combustion engines can also be classified on the basis of FUEL INJECTION

• AIR INJECTION ENGINE - The fuel is injected into the cylinder by a blast of compressed air.  This type of engine was heavy and complicated and now is obsolete.
• AIRLESS (or SOLID or MECHANICAL) INJECTION ENGINE - Fuel is injected into the cylinder, through the fuel valve, by high pressure fuel pump.  At present this is the type of engines which are used.

8.  Internal Combustion engines can be classified on the basis of CHARGING

• NATURAL ASPIRATED ENGINE - In these types of engines, a vacuum is created as the piston moves away from the combustion and as a result draws in fresh charge.  Our car engine is a perfect example of such type of engine. (petrol engines)
• SUPERCHARGED ENGINE - In this type of engine charge is admitted into the cylinder at a pressure greater than the atmospheric pressure.  This high pressure can be produced by a pump or blower or Exhaust Gas Turbocharger.  Our ship's make use of Supercharged engines

9.  Internal combustion engines are classified according to FUEL USED

• HEAVY FUEL OIL ENGINE - These are the engines which can burn high viscosity fuel
• DIESEL OIL ENGINE - These are the engines which can burn Diesel oil
• GASOLINE ENGINE - These are the engines which can burn gasoline as fuel.  These engines can also use kerosene.
• GAS BURNING ENGINE - These are the engines which use gaseous fuels at higher compression.  There are three ways which are adopted to burn these gaseous fuels and as a result these engines are accordingly named.  These engines are:

GAS DIESEL ENGINES - Only air is compressed in these engines.  At the end of compression, gas at high pressure in injected into the cylinder.  With gas, a small amount of fuel termed "pilot fuel" is also admitted into the cylinder to assist in ignition and to cause a smooth and prompt ignition.

DUAL FUEL ENGINE - In these type of engines, gas and air are admitted in the cylinder at the same time and it is the gas/air mixture which is compressed.  At the end of compression, fuel is injected to assist in ignition and cause a smooth and prompt ignition.

Figure shows a typical Wartsila Dual Fuel Engine

HIGH COMPRESSION, SPARK IGNITED GAS ENGINES - In these types of engines, gas and air are admitted in the cylinder at the same time and it is the gas/air mixture which is compressed.  At the end of compression, a spark plug produces a spark which ignites the mixture and causes combustion.

10.  Internal Combustion engines are also classified according Speed

• SLOW SPEED ENGINES - Engines which have rpm less than 300 r.p.m
• MEDIUM SPEED ENGINES - Engines which have rpm ranging from 300-1000 r.p.m.
• HIGH SPEED ENGINES - Engines which have rpm more than 1000 r.p.m.

11.  Internal combustion engines can also be classified according BORE/STROKE RATIO

Figure shows bore and length of stroke

• SQUARE ENGINE - If the bore to stroke ratio becomes 1 i.e if the bore is same as stroke the engine is said to be Square Engine.  In this type of engine Crankshaft web dimensions become less compared to journal and crankpin.

• OVER SQUARE ENGINE (SHORT STROKE ENGINES) - If the bore/stroke ratio is greater than 1, i.e bore diameter is larger than length of stroke.  This allows more valves to be placed in the cylinder head.  These type of engines allow for higher r.p.m and thus more power without excessive piston speed.  These engines have lower friction losses (due to the reduced distance travelled during each engine rotation) and lower crank stress (due to the lower peak piston speed relative to engine speed). Due to the increased piston- and head surface area, the heat loss increases as the bore/stroke-ratio is increased excessively. Because these characteristics favor higher engine speeds, over square engines are often tuned to develop peak torque at a relatively high speed. The reduced stroke length allows for a shorter cylinder and sometimes a shorter connectingrod,generally making over square engines less tall but wider than undersquare engines of similar power.  Source wikipedia

• UNDER SQUARE ENGINE (LONG STROKE ENGINE) -  If the bore/stroke ratio is less than 1 or if the stroke/bore ratio is greater than 1 then the engine is said to be Under square engine.  This means that the length of stroke is greater than the bore.  At a given engine speed, a longer stroke increases engine friction (since the piston travels a greater distance per stroke) and increases stress on the crankshaft (due to the higher peak piston speed). The smaller bore also reduces the area available for valves in the cylinder head, requiring them to be smaller or fewer in number. Because these factors favor lower engine speeds, under square engines are most often tuned to develop peak torque at relatively low speeds.An under square engine will typically be more compact in the directions perpendicular to piston travel but larger in the direction parallel to piston travel.                                                     Source wikipedia

• SUPER LONG STROKE ENGINES - To have better propeller efficiency and better combustion even with lower grade of fuels, lower r.p.m. engines with even longer strokes are gaining popularity.  These engines have stroke/bore ration in the range of 3.

At the end of this blog we can now say that there are 11 Different categories of an Internal Combustion Engine.