Knock - VI. Combustion Chamber Considerations D. Abnormal...

Info iconThis preview shows pages 1–5. Sign up to view the full content.

View Full Document Right Arrow Icon
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 2
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 4
Background image of page 5
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: VI. Combustion Chamber Considerations D. Abnormal Combustion 1) Knock vs. Surface Ignition -— Definition of Knock: Noise associated with abnormal combustion condition generated by unintentional ignition of mixture. — Knock originates from the extremely rapid release of energy in the end gas ahead of the propagating flame. This generates high local temperatures and pressures . - Knock can be generated by " spark knock" and surface ignition. - Surface Ignition: Ignition of the mixture by a source other than the spark plug. Preignition = before spark Postignition = after spark — Spark Knock: autoignition process, a spontaneous ignition of the end gases ahead of the flame. — Preignition is most damaging due to increased gas pressure and temperatures. It can occur earlier and earlier due to increased temperatures (i.e. runaway). - Autoignition can also runaway due to increased temperatures (i.e. knock is unstable) — Autoignition limits compression ratio. — Excessive knock results in structural damage, usually thermal in nature. -- Control: . a - Surface ignition can usually be resolved by design changes : round corners, good valve stem seals, good valve location, deposit reduction (fuel) - Autoignition knock can be controlled by combustion chamber design (e.g. fast burning) and fuel requirements. The goal is to reduce the end gas temperature and pressure and the time required for the flame to reach it. 7» -— Measurement: — Knock is typically measured by a pressure transducer in the combustion chamber. Location in the chamber will affect the results. - Onboard diagnostic monitors for feedback generally employ accelerometers. 2) Fuel Effects and Requirements - The tendency to knock via autoignition is determined by the critical compression ratio. The higher the critical compression ratio, the less the autoignition tendency. — Trends in hydrocarbons: Paraffins: autoignition tendency is reduced by shortening chains or branching Others: Many double bonds (saturated) reduces autoignition tendency - Octane number: — Measurement of the resistance to knock (autoignition) — Increasing the octane number, increases the resistance to knock. — Scale: isooctane = 100 ON n-heptane = 0 ON (additives such as tetraethyl lead are used for ON > 100) — If a fuel under set conditions knocks with the same intensity as a mixture of 90% isooctane and 10% n-heptane, the fuel is given an ON of 90. — Cooperative Fuel Research (CFR) engine is used for testing. - Two sets of operating conditions: Research Octane Number (RON) and Motor Octane Number (MON) - RON: Inlet T = 125 F, N = 600 rpm, Spark Adv. = 13 BTDC MON: Inlet T = 300 F, N = 900 rpm, Spark Adv. = 19—26 BTDC - A/F is adjusted for maximum knock - MON have tougher requirements. -' Antiknock Index = (RON + MON)/2 — Additives to increase ON: 1) Lead compounds (tetraethyl lead) 2) Oxygenates (methanol, ethanol) Normal combustmn Abnormal combustion A combustion process in which a flame front may be started by hot combustion- chamber surfaces either prior to or alter spark ignition, or a process in which some part or all of the charge may be consumed at extremely high rates. A combustion process which is initiated solely by a timed spark and in which the flame front moves completely across the combustion chamber in a uniform manner at a normal velocity. Surface ignition Spark knock“ hot spots—combustion-chamber deposits A knock which is recurrent and repeatable Surface ignition is ignition of the fuel-air in terms of audibility. It is controllable by charge by any hotsurface other than the the spark advance; advancing the spark spark discharge prior to the arrival of the increases the knock intensity and retarding normal flame front. It may occur before the the Spark reduces the intensity. spark ignites the charge (preigm'tion) or after normal ignition (postignition). Runwon Centinuation of engine firing after the electrical ignition is shut off. . * . . . Knocking surface Ignition Nonknocking surface ignition Knock which has been preceded by surface ignition. It is not controlla— ble by spark advance. Surface ignition which does not result in knock. Runaway surface ignition Surface ignition which occurs earlier and earlier in the cycle. It can lead to serious overheating and . structural damage to the Rumble Wild P'ng engine. . . _ . . A low—pitched thudding Knocking surface ignition noise accompanied by Charaaenzéd by one or engine roughness. Probably more erratic sharp cracks. It . . b bl m l f caused by the high rates of 18 pm a y .6 lies.“ t 0 pressure rise associated with early surface ignition from 1 . . . deposit particles very eat y lgmuon or. ' multiple surface ignition. *Knock: The noise associated with autoignition of a portion of the hiel-air mixture ahead of the advancing flame from. Autoignition is the spontaneous ignition and the resulting very rapid reaction of a portion or all of the fuel-air mixture. FIGURE 9-58 Definition of combustion phenomena—normal and abnormal (knock and surface ignition)—in a. spark-ignition engine. (Courtesy Coordinating Research Council.) knock occurrence crank angle Pressure (atn) ‘ZQ ~15 ‘10 -5 G 5 LE 15 29 Crankangle ATC (degrees) ‘L5 5 to 10 kHz Bandpass filter ,... & Knock intensit S U} I ® (J1 Filtered pressure (atn) G S I I r— n—- UT 3 u [\J G -2@ -15 -1@ -5 0 5 10 15 20 ’ Crankangle ATC (decrees) Fig. 2 — Cylinder pressure and band-pass filtered pressure for 94 RON primary reference fuel (PRF) at 1500 rev/min and wide-open throttle. Cycle 2» of data set 262. Spark advance 4O °BTC, equiva— lence ratio 1.03. ' ‘ ‘ n-parafiins 2:: 53.3558 23:5 Isooctane / C I C . l a p_. G _ C _ C Number of carbon atoms ...
View Full Document

Page1 / 5

Knock - VI. Combustion Chamber Considerations D. Abnormal...

This preview shows document pages 1 - 5. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online