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Charakteristiky motorů

Charakteristiky motorů. Rychlostní a úplná charakteristika. Nezaměňovat M jako závisle proměnnou (rychlostní) a nezávisle proměnnou (úplná). Měrná spotřeba paliva b.s.f.c. a výkon motoru. Střední užitečný tlak b.m.e.p. a přebytek vzduchu. Vznětový přeplňovaný motor o zdvihovém objemu 1.2 dm 3.

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Charakteristiky motorů

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  1. Charakteristiky motorů

  2. Rychlostní a úplná charakteristika Nezaměňovat M jako závisle proměnnou (rychlostní) a nezávisle proměnnou (úplná)

  3. Měrná spotřeba paliva b.s.f.c. a výkon motoru. Střední užitečný tlak b.m.e.p. a přebytek vzduchu. Vznětový přeplňovaný motor o zdvihovém objemu 1.2 dm3 Příklady vnějších rychlostních charakteristik motoru

  4. Příklady úplných charakteristik motoru Měrná spotřeba paliva a přebytek vzduchu

  5. Maximální tlak oběhu a mechanická účinnost

  6. Tlak plnicího vzduchu (přeplňovaný vznětový motor)

  7. Engine torque or power is size-dependent; specific work w =W/m depends on pressure upstream of an engine (cylinder charge mass depends on cylinder charge density)  for comparison of different engines their cycle work should be normalized by engine stroke (swept) volume Vz; different works (in cylinder ... indicated, at crankshaft end ... brake mean work, etc.) - marked generally by dot - can be used in the same way Basic engine parameters - mean effective pressure (volume-specific work) (•).m.e.p. ... (brake b or e;, indicated ... i, etc.) mean effective pressure; - no real pressure even if measured in [MPa] or [bar] but volume-specific work [kJ/dm3=MPa]; SI naturally aspirated engine .... b.m.e.p.  1 MPa

  8. Engine speed is size-dependent, as well, i.e., on strokeZ; real inertia caused stress and by contact path determined wear depends rather on velocity not on speed (frequency of cycles);  therefore, mean piston velocityfor comparison of different engines: Basic engine parameters - piston mean velocity Upper limit of medium speed is cca 10 m.s-1 . Engine power is then using engine swept volume using boreD, stroke Z and corresponding piston normal-to-axis area:

  9. Efficiency as output/input. Output is unambiguous - engine work or work/time=power. According to point of measurement  ... e (b) or i or ... Input optional - frequently maximum chemical energy of fuel, „calorific value“ at environment conditions, i.e, -H0, often with slight corrections (see later). Really usable is -G0 but it is unpractical (immeasurable) and f=differences are not big (see $24 ).  therefore, efficiency defined in analogy to thermal one as: Basic engine parameters - efficiency and .s.f.c.

  10. Efficiencies are interesting as an engine economy parameter. They don’t depend on the sort of fuel. More practical using certain fuel is a specific fuel consumption •.s.f.c.= =mp• in g.kW-1.h-1 Basic engine parameters - efficiency and .s.f.c. Calorific value of hydrocarbon fuels (gasoline, diesel oil)  42 ... 43 MJ.kg-1 36% ....  235 g.kW-1.h-1 Efficiencies of a certain class of engines are not very variable. Therefore, it can be used for quick assessment of relation between fuel flow rate and engine power  .m.e.p. or torque from fuel cycle mass. If air-to-fuel A/F ratio is known (stoichiometric air/ hydrocarbon fuel A/F  15), air specific consumption can be easily determined.

  11. Determine brake mean effective pressure [MPa], mean piston velocity [m.s-1] and both nominal and maximum efficencies for Sulzer 14 RTA 96C (see Lecture 1). Calorific value of fuel 42.7 MJ.kg-1. Calculate fuel specific consumption [kg.kW-1h-1] and air flow rate [kg.h-1]. If air-to-fuel A/F ratio is 30, calculate air specific consumption [kg.kW-1h-1] and air flow rate [kg.s-1]. 1‘=0.3047995 m = 12“ 1 (USA) galon=3.785434 dm3 1 lb = 0.4535924 kg 1 psi = 6894.76 Pa 1 HP = 745,7 W 1 k (PS atp.) = 1 kp.m/s = 735.5 W Basic engine parameters – homework 1

  12. Car route fuel consumption dm3.(100 km)-1 (USA other units and reciprocal value - miles/USA gallon) Basic engine parameters - efficiency and .s.f.c. - an example • Examle of “1 liter car” feasibility: • best car diesel engines reach 200 g.kW-1.h-1, i.e, • if density of fuel 1kg.dm-3 (really 0.85kg.dm-3) • 5 kW at 100 km.h-1 wil be just the maximum power •  realistic only if extreme low rolling resistance and air drag is achieved. No margin of power for overtaking, slopes, etc. There is no big margin in efficiency - the best ship medium-speed two-stroke engines feature some 165 g.kW-1.h-1!

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