Kul-14.4100 Internal Combustion Engine Technology Definition & Classification, Characteristics 2015 Basshuysen 1,2,3,4,5
Definitions Combustion engines convert the chemical energy of fuel to mechanical energy as a result of combustion They can be divided into internal and external combustion engines External combustion engines Stirling engines Steam engines Internal combustion engines Reciprocating piston engines Rotary piston engines (Gas turbines) 2
Classifications Basshuysen Figure 2-1 3
Classification 1. Combustion process 2. Fuel 3. Working cycles 4. Mixture generation 5. Gas exchange control 6. Supercharging 7. Configuration 8. Ignition 9. Cooling 10. Load adjustment 11. Application 12. Speed 4
Different piston engines - Basshuysen Figure 2-2 5
Basic structure of reciprocating piston engine Cylinder Piston Piston pin (wrist pin) Piston rings Connecting rod (conrod) Crankshaft Valves (intake & exhaust) Bottom dead center BDC, top dead center TDC Figure: Jussi Nurmiranta 6
Different cylinder arrangements - Basshuysen Figure 2-4 Inline V-engine W-engine Flat-opposed (Boxer) X-engine Different radial engines Double-bank inline H-engine Double-shaft opposed piston 7
4-stroke principle During the intake stroke piston is going down and the intake valves are open. Piston is drawing air (or air-fuel mixture) into the cylinder. During the compression stroke valves are closed and piston is compressing air (or air-fuel mixture). The temperature and pressure of the mixture rise and burning usually begins already at the end of the compression stroke During the expansion stroke gases work against the piston while expanding. Major part of the combustion happens when the piston is still close to TDC. During the exhaust stroke exhaust valves are open and the piston pushes the burned mixture (exhaust gases) out of the cylinder. Working cycle is two revolutions, 720 degrees of crank angle Intake and exhaust strokes together are called the gas exchange 8
Heywood: Figure 1-2 9
2-stroke principle During the compression stroke valves are closed and piston is compressing air (or air-fuel mixture). The temperature and pressure of the mixture rise and combustion usually begins already at the end of the compression stroke. During the expansion stroke gases work against the piston while expanding. Major part of the combustion happens when the piston is still close to TDC. Gas exchange takes place when the piston is close to BDC. Gas exchange begins when the exhaust ports open and the exhaust gases discharge to the exhaust duct (Blow down). While the piston is still moving down the wash ports (intake ports) open and fresh charge of air (or air-fuel mixture) enters the cylinder (scavenging process). Scavenging is succeeded if the pressure in the intake ports is higher than in the cylinder. 10
Heywood: Figure 1-3 Kul-14.4100, syksy 2009 11
Two-stroke scavenging (a) Cross, (b) Loop and (c) Uniflow scavenging Kul-14.4100, syksy 2009 12
SI-engine Spark Ignition engine = SI-engine Also called Otto engine Mixture of air and fuel is ignited with a spark from the spark-plug Flame front propagates through the combustion chamber in 50-60 degrees of crank angle Usually operates with λ=1 (because of threeway catalytic converter) 13
SI-combustion Turbulent pre-mixed flame Figure: Heywood Kul-14.4100, syksy 2009 14
Mixture formation methods in SI-engine Carburetor Intake manifold injection Port fuel injection (PFI) Gasoline direct injection (GDI) Homogenous mixture Stratified mixture: Wall guided, Air guided, Spray guided 15
Mixture formation methods in SI-engine Kul-14.4100, syksy 2009 16
CI-engine Compression Ignition engine = CI-engine Usually called Diesel engine (at least in Europe) Fuel is injected with very high pressure directly into the combustion chamber just before TDC Fuel ignites if the temperature created by compression is high enough for autoignition Burning spray of fuel emerges Combustion is controlled by mixing of air and fuel vapor 17
CI-combustion Figure: Heywood Kul-14.4100, syksy 2009 18
Mixture generation methods in CI-engine Pre-chamber injection Low injection pressure High thermal losses and low efficiency Was used in older passenger car diesel engines Direct injection High injection pressure Combustion chamber in the piston High efficiency Current default combined with exhaust gas turbocharging 19
CI Fuel Injection and Combustion Chamber Pre-chamber, small higspeed engine, simple tappet injector Low injection pressure, High compression ratio, low efficiency, high thermal load. Direct Injection Combustion Chamber, High-Speed Engine, Multi hole injector, High injection pressure, swirl, High efficiency. 20
CI engines Medium-speed with low swirl, one multihole injector Low speed engine: several injectors, typically 2-3, High swirl, exhaust valve in the middle of combustion chamber 21
Supercharging methods - Basshuysen Figure 2-3 22
Turbo-charging 23
Characteristics Cylinder bore [m] Piston stroke [m] Cylinder number Stroke-bore ratio Crank radius [m] Connecting rod length [m] Connecting rod ratio Displacement vol. [m 3 ] Compression volume [m3] Compression ratio Crank angle [deg or rad] D S z S/D r l λ s = r/l V h = πd k2 / 4 * S (one cyl) V c ε (or e) = (V h +V c )/ V c φ or α Engine size always refers to the total displacement volume of the engine (all cylinders included). 24
Schematic of piston engine Basshuysen Fig. 3-1 OT (oberer Totpunkt) = TDC (Top-Dead Centre) UT (unterer Totpunkt) = BTC (Bottom-Dead Centre) 25
Compression ratios - Basshuysen Fig. 3-4 26
Characteristics Rotational speed [r/min (rpm) or rps] n Mean piston speed [m/s] c p = 2sn (c m ) Power [kw] P e Torque [Nm] M Effective mean pressure [bar or Pa] p e Volumetric efficiency λ l (η vol ) Excess-air factor λ (λ tot,λ c ) Spesific fuel consumption [g/kwh] b e Total efficiency η e Fuel net heating value [kj/kg] H u 27
Mean piston speeds of different engines - Basshuysen Fig. 3-5 28
Excess-air factor, relative air to fuel ratio Excess-air factor λ is the ratio of the air mass in the cylinder to the stoichiometric air mass ml ml L λ = = = m m L L L, St m K is the fuel mass injected in the cylinder L St is the requirement of air for stoichiometric combustion [kg/kg], so that the chemical reactions are totally finished. Typically for fuels used in internal combustion engines, such as gasoline and diesel oil, L St is about 14,5 kg/kg L is the mass of air used per the mass of fuel used K St St 29
Excess-air factor, relative air to fuel ratio, equivalence ratio The equivalence ratio Φ is the inverse of the λ. Equivalence ratio is widely used in US literature. Φ = 1 λ Rich combustion: λ < 1 Lean combustion: λ > 1 Total lambda λ tot is based on total air flow trough the engines Combustion lambda λ c is based on the air trapped in the cylinder 30
Fuel properties (gasoline) - Basshuysen Fig. 3-17 31
Power & Torque The brake power at any working point of an engine is calculated from the torque and engine rotational speed P e = M d ω = M 2 Conclusion: increase in the power can be achieved either by increasing the torque or increasing the engine rotational speed d πn 32
Power & Torque - Basshuysen Fig. 3-6 33
Brake mean effective pressure, BMEP Effective mean pressure is a calculated value. It corresponds to a pressure level at which the gases have to work against the piston in order to get the actual work done by the engine or cylinder 2 πdk W = pe s 4 Effective mean pressure describes engine load and the torque that you are able to get out of a certain displacement volume. It is not the average pressure in cylinder! 34
Brake mean effective pressure, BMEP Brake power of an engine is the effective mean pressure multiplied by the displacement volume and rotational speed. P e = p e πd 4 2 k sni i is the number of working cycles per revolution (0,5 for 4-stroke and 1 for 2-stroke engines) 35
36 Actual brake power of an engine is also the torque multiplied by the angular velocity So torque is proportional to the effective mean pressure and the displacement volume n M M sni d p P d d k e e π ω π 2 4 2 = = = i s d p M k e d π π 2 1 4 2 = Brake mean effective pressure, BMEP
Brake mean effective pressure, BMEP 37
Brake mean effective pressure related pressures IMEP = Indicated mean effective pressure = based on the work done by the gases FMEP = friction mean effective pressure BMEP = IMEP-FMEP IMEP(720) = IMEP gross = Indicated mean effective pressure based on gas work over 720 deg CA, normally IMEP= IMEP(720) IMEP(360) = IMEP net = Indicated mean effective pressure based on gas work over 360 deg CA 38
Brake mean effective pressure related pressures PMEP = pumping mean effective pressure = based on the work done by the gases during gas exgange IMEP(720) = IMEP(360) + PMEP More definitions: Heywood Chapter 13.2 39
Volumetric efficiency Volumetric efficiency λ l ( or η vol ) is a measure for the charge cycle and it tells how much fresh charge has been trapped in the cylinder during charge cycle mzges λ l= ρthvh m Zges is the mass of charge air delivered to the cylinder and V H is the total displacement volume of the engine. ρ th is the density of outside air. Volumetric efficiency is a very important value for naturally aspirated SI engines. The better the volumetric efficiency, the greater the maximum torque from an engine. 40
Total efficiency and specific fuel consumption Total efficiency of an engine is the ratio of the brake power and the energy content of fuel flow Pe ηe = m& H K u P e m& K H u is brake power is fuel mass flow is fuel net heating value 41
Total efficiency and specific fuel consumption - Basshuysen Fig. 3-16 42
Total efficiency and specific fuel consumption Specific fuel consumption is the ratio of fuel mass flow and brake power b e = m& P K e Hence we obtain a relation between specific fuel consumption and total efficiency η = e b e 1 H u 43
Mechanical efficiency Mechanical efficiency is the ratio of the brake power flow to power of gases working agaisnt piston η Pe P mech = = i BMEP IMEP Mechanical efficiency is also the ratio of brake mean effective and indicated mean effective pressure 44
Four Stroke SI-Engine Cylinder Pressure, 720 o CA Heywood: Figure 1-8 Kul-14.4100, syksy 2009 45
Four Stroke SI-Engine Cylinder Pressure, 360 o CA Heywood: Figure 1-15 Kul-14.4100, syksy 2009 46
Two-Stroke Engine Cylinder Pressure and Inlet and Exhaust Port Areas, 360 o CA Heywood: Figure 1-16 Kul-14.4100, syksy 2009 47
Specific fuel consumption of a SI engine - Basshuysen Fig. 3-9 48
Specific fuel consumption of a diesel engine - Basshuysen Fig. 3-10 49
BMEP 50
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pv-diagrams: Otto, Diesel, Seiliger, Otto part load, Otto suprecharged 53
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Kul-14.4100, syksy 2009 57
Cylinder pressure cs. pv-diagram Wärtsilä 6L20 cylinder pressure and pv-diagram The real pv-diagram differs from the theoretical diagrams 1. Combustion 2. Heat transfer 3. Exhaust blowdown 4. Gas exchange 58
References Internal Combustion Engine Handbook by Richard van Basshuysen and Fred Schäfer Heywood: Internal Combustion Engine Fundamentals 59