Other internal combustion engines

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Content – Energy sources

 


 

Piston engines

The most common form of internal combustion engines are piston engines based on the “Otto” cycle (gasoline and LPG fuelled engines) and the “Diesel” cycle (diesel fuelled engines).
 




 
Basic components of a piston engine
 
Illustration of main components of an internal combustion engine. (This text is displayed because your browser do not support svg images)
 
1 – Spark plug (ignites the air-fuel mixture in engines based on the “Otto cycle”, engines based on the “Diesel cycle” is self igniting due to pressure and temperature caused by compression)

2 – Inlet cam (opens the inlet valve. The cam profile determines the timing of the valve opening and therefore is one of the main factors affecting the performance and the characteristics of the engine. The cam profile is always a compromise since the optimum valve opening timing is dependant on factors such as rpm and the load. Most engines have a rocker arm that forces the valve to open mounted between the cam profile and the top of the valve stem (not shown on this illustration))

3- Valve spring (the valve spring forces the valve to close at sufficient speed (some high rpm, high performance engines have instead of a valve spring, a leverage system that closes the valve (desmodromic). This system is more mechanically complicated than a spring fitted system but the other hand allows a higher degree of control and avoids the issue with springs that float at high rpm, meaning that the spring is not able to release fast enough compared to the rpm.)

4 – Inlet (leads the air or air-fuel mix into the cylinder. The shape and size have influence on the performance and the characteristics of the engine)

5 – Inlet valve (Allows air or air-fuel mix to be sucked into the cylinder or combustion chamber. If the engine is supercharged the air or air fuel mixtures is pushed into the cylinder or combustion chamber by the overpressure caused by a compressor and turbo charger.)

6 – Piston (the shape of the piston head and the cylinder head is what makes the combustion chamber which has a great influence on how the air-fuel mixture behaves and how the combustion takes place.)

7 – Piston rings (their primary function is to make the seal between the piston and the cylinder walls)

8 – Cylinder or engine block

9 – Piston rod or connecting rod (transfers the forces created by the combustion from the piston to the crank shaft.)

10 – Crank pin

11 – Crank case

12 – Crank shaft

13 – Crank shaft web

14 – Crank shaft counter weights (to balance and compensate for the uneven forces on the crank shaft from the pistons.)

15 – Piston pin

16 – Liquid cooling

17 – Exhaust (the shape, length and diameter of the exhaust system influence on the performance and the characteristics of and the noise from the engine. The exhaust system in modern transportation vehicles normally have a catalyst for reducing the amount of harmful emissions such as Nox, hydrocarbons and sot.)

18 – Exhaust valve (allows the exhaust gases to flow out of the cylinder.)

19 – Cylinder head

20 – Cam shaft (most modern high performance engines have overhead mounted twin cam shafts)

21 – exhaust cam (opens the exhaust valve. The cam profile determines the timing of the valve opening and therefore is one of the main factors affecting the performance and the characteristics of the engine. The cam profile is always a compromise since the optimum valve opening timing is dependant on factors such as rpm and the load. Most engines have a rocker arm that forces the valve to open mounted between the cam profile and the top of the valve stem (not shown on this illustration))

Two stroke and four stroke piston engines.

The number of strokes describes the number required for one piston to complete a full cycle. One stroke is the movement from TDC (Top Dead center) to BDC (Bottom Dead Center) or vice versa, ie. The position where the piston arrives at a full stop, before reversing the movement.
 

Top dead centreBottom dead centre
Illustration of top dead centre of four stroke internal combustion engine . (This text is displayed because your browser do not support SVG)Illustration of bottom dead centre of four stroke internal combustion engine . (This text is displayed because your browser do not support SVG)

 

Four stroke internal combustion engine

1st Stroke – Intake (air or air fuel mix)
2nd Stroke – Compression (compression and ingnition. Ignition either by spark or by fuel injection)
3rd Stroke – Combustion followeed by expansion (Power or working stroke) (crank shaft is rotated by pstion mlovement)
4th Stroke – Exhaust (Combustion gases expelled)

The four-stroke engine is by far the most common for powering road vehicles.

Stroke 1 – fuel and air intake

Stroke 2 – compression

Illustration of stroke 1 of four stroke internal combustion engine . (This text is displayed because your browser do not support SVG)Illustration of stroke 2 of four stroke internal combustion engine . (This text is displayed because your browser do not support SVG)

Stroke 3 – combustion and expansion

Stroke 4 – exhaust

Illustration of stroke 3 of four stroke internal combustion engine . (This text is displayed because your browser do not support SVG)Illustration of stroke 4 of four stroke internal combustion engine . (This text is displayed because your browser do not support SVG)

Two stroke internal combustion engine

1st Stroke – Combustion (power working stroke), expansion, scavenging (exhaust) and intake
2nd Stroke – Scavaenging continues (exhaust) and compression

 

Stroke 1a – combustion and expansion

Stroke 1b – expansion, intake and scavenging

Illustration of stroke 1a of two stroke internal combustion engine . (This text is displayed because your browser do not support SVG)Illustration of stroke 1b of two stroke internal combustion engine . (This text is displayed because your browser do not support SVG)

Stroke 2a – scavenging and compression

Stroke 2b – compression

Illustration of stroke 2a of two stroke internal combustion engine . (This text is displayed because your browser do not support SVG)Illustration of stroke 2b of two stroke internal combustion engine . (This text is displayed because your browser do not support SVG)

The engines are in addition to the number of strokes categorised according to the thermodynamic process they are working to. The “Otto cycle” or the “Diesel cycle”.

The Otto cycle

1-2 Adiabatic compression
2-3 Combustion (temperasture increase) at constant volume
3-4 Adiabatic expansion
4-1 Gas exchange (heat removal) at constant volume.

Ideal (theoretical) thermal efficiency
 
eta_0= \dfrac{w_0}{Q_t}= \dfrac{Q_t-Q_b}{Q_t}=1- \dfrac{Q_b}{Q_t} = 1- \dfrac{c_v(T_4-T_1)m}{c_v(T_3-T_2)m} = 1-\dfrac{(T_4-T_1)}{(T_3-T_2)}
 
c= Specific heat capacity for heating at constant volume (J/KgoK)
m = Charge (Kg)

Since: PV/T = Constant and during the adiabatic expansion (no heat transfer)
PVK = Constant

and during polytropic process
Pen = Constant
κ = adiabatic factor
n = Polytropic factor

The ideal (theoretical) thermal efficiency may then be expressed:
 
eta_0=1- \dfrac{V_2}{V_1}
 
V1 = Cylinder volume above the piston top at BDC XZ
V2 = Cylinder volume above the piston top at TDC
 
\varepsilon_n= \dfrac{V_1}{V_2}= Nominal compression ratio
 
eta_0=1- \dfrac{1}{\varepsilon^{\varkappa-1}_n}
 
The ideal thermal efficiency for the Otto process will therefore increase with increased compression ratio.

This will aslo be the case in an actual application however there are various factors that limit the possibility and the desire for a high compression ratio.

The two main limiting factors related to increasing the compression ratio are:

  • The risk for self-detonating which increases by increased compression ratio
  • The environmental effect, harmful emissions increases by increased compression ratio.

The Diesel cycle

1-2 Adiabatic compression
2-3 Combustion (temperasture increase) at constant pressure
3-4 Adiabatic expansions
4-1 Gas exchange (heat removal) at constant volume.

The main difference compared with the Otto cycle is that during the Diesel cycle the fuel is gradually injected during combustion such that the duration of the combustion takes longer time. Therefore the ideal (theoretical process) is based on combustion at constant pressure.

Ideal (theoretical) thermal efficiency:
 
eta_0=1- \dfrac{1}{\varepsilon^{\varkappa-1}_n} times \dfrac{rho^\varkappa-1}{\varkappa(rho-1)}
 
κ = Cp/Cv = Adiabatic factor
εn =V1/V2 = Nominal compression ratio
ρ = V3/V2 = Volume differential during combustion

The ideal thermal efficiency for the Otto process will aslways be highert than for the Diesel process at the same compression ratio since that;
 
\dfrac{rho^\varkappa-1}{\varkappa(rho-1)}
 
always will be larger than 1. However since the Diesel process in most cases do operate at higher compression ratios and since the temperature at the end of the process is lower and thereby less heat transfer, the ideal thermal efficiency for the Diesel process is higher than for the Otto process assuming the max pressure and the applied heat is the same.

Actual processes The actial effieciency is less than the efficiency of the ideal (theoretical) processes. The actual efficiency is primarily affected by the following factors comaored to the ideal processes:

  • The specific heat capacity for air is not constant as assumed in the ideal processes but actually increases with the temperature.
  • The adiabatic factor will decrease with increased temperature.
  • The “air” will in practical terme not be pure during the full cycle. Combustion gases with higher specific heat capacity will be present after the combustion have taken place
  • Compression is polytropic and not adiabatic since heat excahge will occure through the cylinder walls to the cooling media.
  • Ignition does not take place exactly at TDC but somewhat earlier.
  • The combustion do not take place with constant volume (Otto) or constant pressure(Diesel)
    • Otto process – The combustion take some time with the result that there will be volume changes and a reduced (compared to the ideal process) maximum combustion pressure as a result.
    • Diesel process – Since the initial part of the combustion will take place with a very high speed and the pressure will increase.
  • Some of the fuel will not be fully combusted (incomplete combustion) such that not all the energy contained in the fuel will be converted to heat.
  • The expansion is not adiabatic.
    • Otto – Due to high temperatures will CO2 and vaporised water will dissociate. During expansion when the temperature falls the dissociated components will again be bonded with the consequence that heat is released (same effect as if the combustion was prolonged).
    • Diesel – Some of the combustion continuous into the expansion stroke after the fuel injection have ended.
  • Exhaust valves or passages do not open exactly at BDC but somewhat earlier.