THE first commercially successful internal-combustion engine was patented in 1858 by Belgian Étienne Lenoir. Since then, there’s been an unending quest to increase combustion efficiency so that more power can be produced while using less fuel. Automakers employ advanced technologies to gain even fractional improvements in efficiency.
However, the quest for lower fuel consumption and emissions amid the demand for increased power and torque does come with the risk of reaching the knock limit; push the boundaries too far and the engine will destroy itself in minutes.
Knock-knock
Basics of petrol-engine combustion
In a four-stroke petrol engine, the intake stroke provides each cylinder with oxygen in the form of fresh air, either at atmospheric pressure (naturally aspirated) or pressurised (turbo- or supercharged). Petrol is added to the air stream in a modern engine at the ideal ratio (also called stoichiometric ratio) of approximately 1 part fuel to 14 parts air, by either port injection (upstream of the intake valve) or injecting it directly into the combustion chamber (direct injection). During the compression stroke, the air-fuel mixture is compressed and ignited by the spark plug just before the piston reaches its highest position, or top-dead centre. A controlled burning process then releases the fuel’s chemical energy, with the resultant combustion pressure pushing down on the piston during the power stroke (see graph to the right for a normal combustion pressure trace).
What is knock?
The combustion process in a petrol engine is sometimes incorrectly referred to as an explosion instead of a controlled burning process. Normally, a flame front will start at the spark plug and spread gradually throughout the entire combustion chamber and consume the air-fuel mixture. During a knock event, the air-fuel mixture ignites spontaneously if certain thresholds are breached (see The knock limit on page 117) and then combustion closely resembles an uncontrolled explosion. Normally, knock will be triggered by the pressure wave that results from the normal spark-initiated combustion, but it occurs in a different location within the combustion chamber. Carbon deposits can also lead to hot spots in the chamber that trigger knock.
In older vehicles, knock could be identified by a pinging sound from the engine. These extreme pressure waves in the combustion chamber have the potential to destroy the metal walls (piston and cylinder) of the combustion chamber in minutes (see graph to the left for a pressure trace during a knock event).
The knock limit
In theory, the combustion process is more efficient at higher compression ratios (up to a point and ignoring pumping and heat losses). Efficiency would have been further enhanced if it were possible for the energy release to instantly take place during the top-dead-centre position of the piston in the cylinder during combustion. However, the problem is that the air-fuel mixture has a tendency to self-ignite (or auto-ignite) if the temperature and pressure rises above a threshold (called the knock limit) during compression. The following criteria can result in the air-fuel mixture auto-igniting:
• The compression ratio is too high;
• The boost pressure is too high (in turbo- or super-charged engines);
• The petrol-octane rating is too low (see side bar on petrol octane number versus performance);
• The ignition timing is too far advanced;
• Or the fuel mixture is too lean.
Future
When an engine runs lean (at an air-fuel ratio of higher than 14:1) in combination with stratified injection under part-load conditions, gains in efficiency are realised owing to fewer pumping losses. Another innovation in petrol-engine technology is the absence of spark plugs, or homogenous charged-compression ignition (HCCI) units.
Automakers currently experiment with knock in the same way as diesel combustion. The major challenge that lies ahead is controlling auto-ignition without damaging the combustion chamber.
Calibrating knock-control strategies
Manufacturers spend vast sums during engine development to optimise combustion. An important aspect of this process is knock calibration.
Test Setup
An engine is set up on a test stand in a dynamometer test cell. Apart from the regular instrumentation of a dyno test cell, the engine is also instru-mented with in-cylinder pressure transducers for each cylinder. These pressure signals are used as input to signal analyser equipment such as the AVL Indimaster, which is capable of the real-time plotting of pressure traces against the crank angle for each combustion event.
Pages: 1 2