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Двигатели Стирлинга

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Двигатели Стирлинга

Сайт о двигателях Роберта Стирлинга. История, описание, конфигурация, принцип работы... А также о других двигателях, тепловых машинах, двигателях внешнего и внутреннего сгорания, новинках в машиностроении. More »

Двигатели Стирлинга

Сайт о двигателях Роберта Стирлинга. История, описание, конфигурация, принцип работы... А также о других двигателях, тепловых машинах, двигателях внешнего и внутреннего сгорания, новинках в машиностроении. More »

 

Архивы рубрики: An Introduction to Engine Testing and Development

Calibration Issues

It is important to establish a running-on-road footprint of the key speed and load points. Legislated emission points hold priority, of course, as do drivability, performance, power, and fuel economy. Calibration must be an exercise in compromise. For example, when reviewing the required ignition timing for a given throttle and speed load point, we would

5000 (1 Crank Angle < ‘ATDQ Pre-Ignition

Pre-ignition, or runaway ignition, is ignition in the combustion chamber prior to spark discharge. Referring to Figure 8.56, where will the onset of NOx start? As the pressure within the cylinder rises rapidly, so will the temperature. It will rapidly reach the point where the energy of the bum will disassociate the triple bond nitrogen

Causes of Partial Burns

The following are causes of partial bums, where the bum is initiated but is extinguished rapidly (Figure 8.51): • Bum duration is too long — Insufficient charge motion — Low compression pressures — Excessive dilution (residuals, air, EGR) • Spark timing is too retarded • Cylinder contents are not well mixed Irregular misfires due to

Causes of Misfire

The causes of misfire may be as follows: • Insufficient ignition energy • Conditions at the spark plug at the time of spark that are not conducive to ignition: Normal Misfire   П: Я A.   Figure 8.49 A pressure volume diagram indicat­ing misfire.   U   Cylinder Volume (ce)   COV of 1MEP =

Incomplete Combustion

With incomplete combustion, misfires and partial bums occur when flame propaga­tion either is never properly initiated or fails to propagate fully across the combustion chamber prior to the exhaust valve opening. For example, with flame initiation, misfire occurs due to poor spark initiation. Likewise, misfire occurs if the rate of conductive heat losses exceeds the

Abnormal Combustion

With abnormal combustion, an incomplete bum (misfires and partial bums) can occur. Other problems become apparent as pre-ignition and knock.

How Is Combustion Variability Quantified?

The most common methods to quantify cycle-to-cycle and cylinder-to-cylinder vari­ability include the standard deviation of IMEP and the standard deviation of revolutions per minute. • Standard deviation of IMEP quantifies how widely values are dispersed from the mean, that is, STDEV of IMEP = ny№-1MEP>2 Hi <"-» Where i is the sample of interest and

Impact

Combustion variability impacts engine performance at all operating conditions. Idle instability typically is driven by variations in fuel flow and exhaust residuals. Part­load variability is driven by fuel flow variations and EGR. Wide open throttle (WOT) combustion instability typically is dictated by variations in airflow. Note that in the example given in Figure 8.37, there

Causes

Combustion variability may be caused by the following: • Mixture motion at the location and time of spark • Variation in the amount of air and fuel inducted in each cycle • Mixing of the fuel and exhaust residuals • Fuel preparation (droplet size, cone angle, targeting, swirl [barrel and tumble]) • Excessive dilution, exhaust

How Does Combustion Variability Manifest Itself?

The following are some signs of combustion variability: • Engine roughness • Cyclic and cylinder-to-cylinder variations in torque and engine speed • Compromised torque/power • Lower resistance to knock • Efficiency losses • Higher emissions • Lower fuel economy • Compromised dilution tolerance • Compromised spark timing (injection point diesel)