A DETAILED overview of KNOCK and PRE-IGNITION - BOOST SCHOOL #7

what is up engine heads welcome to another episode of boost school the youtube equivalent of a university course on forced induction made possible by the good people over at aem performance electronics now today we're talking about probably the number one killer of boosted engines and that of course is knock we're going to understand what knock is how it differs from pre-ignition how to avoid it and if it happens how to control it so what is knock well the simplest possible explanation of knock is that it's abnormal combustion so combustion the way it shouldn't be happening but to truly understand knock we have to dive deeper than that and understand what is actually happening inside the engine so here we have an engine cylinder this cylinder is on the compression stroke so the piston is moving upward both the intake and the exhaust valves are closed and the piston is compressing the air fuel mixture inside it now the spark plug is actually going to fire before the piston reaches its topmost position of travel top dead center or tdc the spark plug must fire before the piston reaches tdc to ensure that maximum combustion pressure builds up by the time the piston actually starts traveling downward now the important thing to understand here is that combustion inside an engine is not an instant explosion or bank as it's sometimes described instead it's an evenly propagating flame front now depending on the type of fuel the airfue ratio and other factors the typical flame speed of this flame front inside a gasoline engine is between 15 to 45 meters per second which is around 50 to 150 feet per second or between 50 to 160 kilometers per hour in contrast to this the flame speed of a dynamite explosion is around 8 800 feet per second which is around 9500 kilometers per hour so combustion is not a violent explosion but it still has a pretty fast flame front but there's something else that's fast inside the engine and that's the piston for example the 1.6 liter engine in my toyota mr2 project car has a stroke of 77 millimeters this means that at six and a half thousand rpm the average piston speed is around 17 meters per second so as you can see the speed of the piston is comparable to the speed of the combustion but it is the combustion pressure that pushes the piston downward and to ensure that the piston receives the maximum amount of combustion pressure and transfers the maximum amount of combustion pressure onto the crankshaft we have to make sure that the piston is in the correct position to receive the combustion pressure when it's at its maximum to achieve this we usually want the piston to be somewhere between 15 to 18 degrees of crankshaft rotation after top dead center when maximum combustion pressure builds up to ensure this happens we're going to fire the spark plug early to give the combustion enough time to build up pressure and reach its peak at the correct time this early firing of the spark plug is called ignition advance and on gasoline engines ignition advance is typically around 8 to maybe 15 degrees before top dead center at idle and somewhere between 30 to 40 degrees before top dead center at wide open throttle and high rpm so in the beginning of the video we said that knock is abnormal combustion so first let's see what's normal combustion normal combustion occurs like this the piston is traveling upward is compressing the air fuel mixture at some point before the piston reaches tdc the spark plug fires and it initiates the combustion event the combustion spreads evenly outward from the spark plug until most or all of the air fuel mixture is burnt off now knock occurs like this the beginning is the same the piston is moving upward it compresses the air fuel mixture the spark plug fires and when the spark plug fires the combustion event starts now as the combustion spreads outward it builds pressure and it exerts pressure on the still unburned air fuel mixture outside the boundary layer of combustion at some point one or more pockets of the air fuel mixture outside the flame front of the combustion spontaneously ignite when this happens this is called knock now knock is also called detonation detonation is characterized by supersonic shock waves meaning that they travel faster than the speed of sound and it is these shock waves that resonate throughout the cylinder and if they're strong enough they create the characteristic knocking sound that we can hear sometimes even outside the engine it is also these shockwaves that have the potential to damage things around them in contrast to detonation normal combustion is also called deflagration unlike detonation deflagration has subsonic flame fronts which travel below the speed of sound and they do not have destructive potential instead they simply exert pressure on the things around them deflagration increases cylinder pressure in an even predictable and controllable manner in contrast to this detonation or the self-ignition of the air fuel mixture is unpredictable and chaotic and it causes brief but very intense and potentially damaging spikes in cylinder pressure now knock differs from pre-ignition in the timing of when it occurs knock is the spontaneous self-ignition of the air fuel mixture after the spark plug fires pre-ignition is a spontaneous self-ignition of the air fuel mixture before the spark plug fires so both knock and pre-ignition can have similar damaging consequences for the engine but it's usually pre-ignition that causes more damage more rapidly because pre-ignition happens on the compression stroke which means that the piston is actually heading head first into the shock waves and increased temperatures of pre-ignition whereas during knock the piston is actually running away from the damaging effects of knock because knock is happening on the power stroke damage from very light and brief knock is usually non-existent the first signs of physical damage from knock are usually tiny little spots that you can see on the surface of the piston crown they look like erosion or even sand blasting in some cases and leave behind a slightly rough piston crown if not persists long enough it's going to increase cylinder pressure and temperature to the extent that piston ring ends are going to come into each other they're going to butt up the piston ring will have nowhere left to go and this is going to increase the outward pressure of the piston ring against the cylinder this is going to increase the friction between the ring and the cylinder and this is going to increase the heat and the load on the ring glands eventually the ring glands are going to break which is going to cause a loss of compression and engine failure in the most severe of cases knock and pre-ignition can actually melt holes through pistons although not can do this too pre-ignition is the more common culprit because the piston is working against it's going straight into the increased temperatures caused by pre-ignition it's exposed to these severe conditions longer during pre-ignition than during knock also because the piston is on the compression stroke it's working against the forces of pre-ignition pre-ignition is also the more common culprit when it comes to bending connecting rods so why do knock and pre-ignition occur well the simplest answer is because temperatures inside the cylinder get too high this is a problem in a gasoline engine because a gasoline engine is compressing both air and fuel and if cylinder temperatures get too high this can lead to a spontaneous self-ignition of the air fuel mixture aka knock this is also the reason why knock really isn't an issue in diesel engines diesel engines compress only air and add fuel only when the air is hot enough to ignite the fuel this is why diesel engines don't need spark plugs so the best way of preventing knack is to prevent the air coming into the engine from getting too hot well that's an easy fix take your race car and move to siberia and you can forget all about knack well yes an engine that has high chances of experiencing knock is less likely to knock when ambient temperatures are sub-zero but of course it's stupid to let the weather decide when you drive your car fortunately there is better ways of avoiding knock one of the easiest ways to avoid knock is to simply use a higher octane fuel a higher octane fuel offers more resistance to knock which means that it's less likely to self-ignite than a fuel with a lower octane number another good way of avoiding knock is choosing the correct compression ratio for your engine when we compress fluids such as air we increase their temperature when we compress air we bring the molecules closer together when they're closer together they bump against each other more often increasing friction and thus the temperature of the air the ratio between your largest cylinder volume when your piston is at bottom dead center and your smallest cylinder volume when your piston is the top dead center the ratio between these two volumes is your compression ratio your static compression ratio the higher this number the greater your compression ratio the more your piston is compressing the air fuel mixture the higher the compression ratio the higher the potential for knock now adding forced induction to the engine further increases the potential for knock because turbos and superchargers essentially compress the air they stuff more air into the same space by compressing it and then you can add more fuel and create a more powerful combustion with higher combustion pressures and create more power but because turbos and superchargers essentially compress the air they're actually adding heat into the system and further increasing the potential for knock this is why engines with forced induction are always going to run a lower compression ratio than an identical engine that's naturally aspirated running a lower compression ratio decreases air temperatures and leaves some room for the compressed air coming from the turbo or supercharger this is also why proper intercooling is very important for engines with forced induction an intercooler regardless of whether it's water to air or air to air relies on the process of heat exchange to remove heat from the compressed air coming from the turbo or supercharger this reduces temperature of the intake air and greatly reduces the chances for knock now you could call compression ratios in intercooling passive ways of reducing the chances for knock but there's also active ways of avoiding dock and probably the best and easiest one is water meth injection directly into the intake air stream for example aem's new and improved v3 water meth injection kit is discreet tasteful easy to install and easy to set up and it enables you to increase power outputs by as much as 20 now methanol is a high octane fuel that is very resistant to knock and together with water it absorbs heat out of the system allowing you to run more boost pressure than you could ever before as an added bonus water meth injection actually prevents carbon buildup thus reducing the chances of hot spots in the future so increasing boost pressure increases the compression of the air which increases the chances of knock also running a higher compression ratio increases the temperatures of air which increases the chances of knock on the other hand we have things that reduce the chances of knock which is higher octane fuel better intercooling and water meth injection so does this mean that the best way of you know avoiding knock is running low boost low compression getting a giant intercooler running uh you know high octane fuel and making sure your water methane never goes empty well yes you can do that and have a joint safety margin but that's not the point the point of engines is to make power to be efficient and the reality is that engines make most power right at the very edge of knock right before knock actually begins this is where most most power lies which means that to make power you need to constantly dance at the end of knock but that's risky and stupid well well yes it was risky 30 years ago but not so much today back in the day the way to safeguard against knock was to run very low compression ratios and you know run a giant safety margin which prevented knock from happening this is because back in the day engines had no way to actively respond to knock if knock happened the engine couldn't do anything about it but already in the 80s nox sensors began to be commonplace on engines a knock sensor is essentially a microphone and it listens to the frequency of knock inside your engine when it hears knock it relays a signal to the ecu and the ecu can do something about it if knock is detected the issue is usually going to timing and or add fuel into the system so why does retarding the ignition timing stop knock well let's imagine we fire the spark plug at 35 degrees before top dead center let's also imagine that maximum combustion pressure builds by the time the piston is 17 degrees after top dead center let's imagine knock occurs the knock sensor detects it sends a signal to the ecu and now the ecu decides to stop knock that next time for the next combustion cycle it's going to fire the spark plug later it's going to timing and fire the spark plug at 30 degrees before top dead center because we fired the spark plug later it also means that maximum combustion pressure is going to build later let's imagine that when we fire the spark pocket 30 degrees before top dead center maximum combustion pressure builds by the time the piston is 20 degrees after top dead center so this time the piston is further down the bore when maximum combustion pressure builds up this means that this time around combustion pressure is building in a larger space a larger volume and when we increase volume we reduce pressure this means that we're also reducing the pressure exerted on the unburnt air fuel mixture outside of the combustion frame front this also means that we're reducing the temperature of the unburned air fuel mix which reduces the chances of it spontaneously self igniting of course reducing combustion pressure reduces power but it's a small price to be paid for saving your engine and on top of this modern ecu's are really clever when they detect knock they timing a lot to play it safe and then they incrementally again each combustion cycle increase timing until they detect the faintest trace of knock then they reduce timing again so modern ecu's are actually constantly dancing around the edge of knock saving the engine and squeezing the maximum power out of it if retarding the timing doesn't work the ecu can also increase the amount of fuel injected into the engine thus creating a richer air fuel mixture a richer air fuel mixture helps reduce cylinder temperatures and thus prevent knock this is also the reason why air cooled engines must run richer than liquid cooled engines air cooled engines do not have coolant circulating through them so they must rely on a richer air fuel mixture to keep cylinder temperatures in check now although early ecu's could detect knock and could respond to it they were a bit slow so over time their weighted responses could spell disaster for the engine but modern ecu's are a different story for example i'll be running an aem infinity series 5 standalone ecu in my toyota mr2 uh turbo project build now this is a 200 kilohertz processor that can process 400 million instructions per second this means that it's more than fast enough to detect knock and fix ignition timing before the next compression stroke even at high rpms on top of that it comes with two noc sensing circuits so you can run two knock sensors and it also works with piezoelectric and flat response knock sensors right out of the box so that's pretty much it for today's video these are some of the key points when it comes to knack but knock and its relationship with other things inside your engine is a very rich and very complex topic that will be definitely exploring a lot more in the future of boot school both in theory and in practice so yeah as i said that's it for today uh as always thanks a lot for watching and i'll be seeing you soon with more fun and useful stuff on the d4a channel

Channel: driving 4 answers

Views: 425,213

Keywords: detonation, pre ignition, pre-ignition, knock, knocking, engine knock, ignition advance, knock sensor, knock control, ecu, engine tuning, 1000 hp, tesla hp, ice, engine detonation, engine damage, piston damage, broken piston, connecting rod, engine failure, advancing ignition, retarding ignition, piston rings, boost, boosted, turbo, supercharger, turbocharger, turbocharged, effects of pre-ignition, how to fix pre-ignition

Id: G5bJlFHKOX0

Length: 16min 14sec (974 seconds)

Published: Sun Jun 20 2021