12 Engine Operating Characteristics 473y2q

TOPIC

10

Engine operating characteristics Sections 15.1, 15.2, 15.3, 15.5

Spark Ignition Engines

Indicated and brake power and mep for SI engine Pig  Pb  Pf 1 sfc   f QHV

mep   f v QHV a,i  F A 

Wide-open-throttle characteristics for SI engine. B/S = 96.8/86, rc = 8.6

Mean effective pressure vs. speed • Bmep curve reflects the variation with speed of v, m, and f,i • Maximum volumetric efficiency occurs in the middle speed • Mechanical efficiency decreases as speed increases • Fuel conversion efficiency increases as speed increases due to the decreasing importance of heat transfer per cycle

: Volumetric efficiency vs. speed

Operating variables that affect SI engine performance, efficiency and emissions • Spark timing • Equivalence ratio • EGR • Speed and load

Spark timing

Maximum brake torque timing gives maximum brake power and minimum brake specific fuel consumption

Effect of spark advance on brake torque at constant speed and A/F

Spark timing

•

As speed increases the spark must be advanced to maintain optimum timing because the duration of the combustion process in crank angle degree increases

•

As load and intake manifold pressure are decreased, the spark timing must be further advanced to maintain optimum engine performance Brake torque vs. spark advance at various speeds

Spark timing

Predicted variation in bsfc with spark retard at several different part-load conditions

Spark timing

Variation of exhaust NO concentration with spark retard Left-hand end of curve corresponds to MBT timing for each A/F

Equivalence ratio

Equivalence ratio

Variation of brake specific HC and NOx emissions and fuel consumption with fuel/air equivalence ratio at constant speed and load

Exhaust gas recycle

Variation of exhaust NO concentration with percent recycled exhaust gas (EGR) Spark-ignition engine, 1600 rpm, MBT timing

Exhaust gas recycle

Effect of EGR on brake specific fuel consumption at constant bmep and speed, sotichiometric mixture, and various burn duration (0-100 percent). Prediction from thermodynamic-based cycle simulation

Effect of EGR on fuel consumption •

•

Improvement in fuel consumption with increasing EGR is due to three factors: 1)

reduced pumping work as EGR is increased at constant brake load (fuel and air flows remains almost constant; hence intake pressure increases);

2)

reduced heat loss to the walls because the burned gas temperature is decreased significantly;

3)

a reduction in the degree of dissociation in the high-temperature burned gases which allows more of the fuel’s chemical energy to be converted to sensible energy near TDC

The first two of these are comparable in magnitude and each is about twice as important as the third

Load and speed: performance map

Wide-open-throttle performance curve

Performance map for 2-dm3 four-cylinder fast burn SI engine showing contours of constant bsfc in grams per kilowatt-hour. Upper envelope of the map is the wideopen-throttle performance curve

Performance map: curve (e)

Shape of wide-open-throttle bmep curve (e) was discussed earlier

Performance map: direction (a)

Decreasing speed at constant load increases bsfc due primarily to the increasing importance of heat transfer per cycle (which decreases fuel conversion efficiency) Friction decreases, increasing mechanical efficiency, but this is secondary. Mixture enrichment required to avoid misfire at low engine speed contributes too

Performance map: direction (b)

Increasing speed at constant load increases bsfc due primarily to the increasing friction mep at higher speeds (which decreases mechanical efficiency). While fuel conversion efficiency increases as speed increases, but friction increase dominate

Performance map: direction (c)

Increasing load at constant speed from the minimum bsfc point increases bsfc due to the mixture enrichment required to increase torque

Performance map: direction (d)

Decreasing load at constant speed increases bsfc due to the increase magnitude of friction (due to increased pumping work), and increasing importance of heat transfer (which decreases fuel conversion efficiency)

Load and speed: affect on emissions

Compression Ignition Engines

Indicated and brake power and mep for CI engine 1 sfc   f QHV

mep   f v QHV  a,i  F A  fmep m  1  imep

 f ,b  im Indicated and brake power, mep and sfc for naturally aspirated direct-injection diesel engine. B/S = 115/135, rc = 16

Effect of load and speed

Performance map of a naturally aspirated medium-swirl DI diesel engine. B/S = 102/100, rc = 18

Effect of load and speed

Performance map of a naturally aspirated IDI swirl-chamber diesel engine. B/S = 76.5/86.4, rc = 23

DI vs. IDI

Factors which improve the indicated efficiency of naturally aspirated small DI diesel combustion system relative to IDI swirl-chamber combustion system, as a function of A/F or load

Effect of load on NOx and HC

Smoke and particulate

Effect of injection timing

Engine performance summary •

Within a given category of engines (e.g., naturally aspirated four-stroke SI engines) the values of maximum bmep, and bmep and mean piston speed at maximum rated power, are closely comparable. Within an engine category where the range in size is substantial, there is an increase in maximum bmep and a decrease in minimum bsfc as size increases due to the decreasing relative importance of friction and heat loss per cycle. There is also a decrease in mean piston speed at maximum power as engine size increases. Note the higher bmep of naturally aspirated SI engines compared to equivalent NA diesels due to the fuel-rich operation of the former at wide-open throttle

Engine performance summary •

Two-stroke cycle spark-ignition engines have significantly lower bmep and higher bsfc than four-stoke cycle SI engines

Engine performance summary •

The effect of increasing inlet air density by increasing inlet air pressure increases maximum bmep values substantially. Turbocharging with after-cooling gives increased bmep gains relative to turbocharging without after-cooling at the same pressure level. The maximum bmep of turbocharged SI engines is knock-limited. The maximum bmep of turbocharged CI engines is stress-limited. The larger CI engines are designed to accept higher maximum cylinder pressures, and hence higher boost

Engine performance summary •

The best efficiency values of modern automobile SI engines and IDI diesel engines are comparable. However, the diesel has a significant advantage at lower loads due to its low pumping work and leaner air/fuel ratio. Small DI diesels have comparable (or slightly lower) maximum bmep to equivalent IDI diesels. the best bsfc values for DI diesels are 10 to 15 percent better, however

Engine performance summary •

In the DI diesel category (which is used over the largest size range – less than 100 mm bore to almost 1 m), maximum bmep and best brake fuel conversion efficiency steadily improve with increasing engine size due to reduced impact of friction and heat loss per cycle, higher allowable maximum cylinder pressure so higher boost can be used, and (additionally in the larger engines) through turbocompounding