Ok I understand what a knock sensor is and its function. Here is where I am lost. If knock sensors are designed to allow the use of lower octane fuels in a higher compression engine by retarding the timing to compensate at the expense of power, wouldn't runing a higher octane fuel in an engine with a knock sensor improve performance by allowing the timing to stay more advanced?

 

Yes, but it advances spark farther before TDC to ignite mixture before it can self ignite from compression heat.

Yes, if your engine system is using knock sensor consistently on regular 87 octane then running a higher octane would improve power and MPG both.


Octane is a heat rating, lower octane will self ignite at a lower temp, and a gallon of regular gas and a gallon of 93 octane have the same amount of energy, so normally running premium fuel doesn't change anything, i.e. power or MPG, unless there is a problem running regular gas

Diesel engines are built for self ignition, thats how they run, self ignition from high compression heat, and why they sound "pingy"
Gasoline engines are damaged by pings/knocks, it pits the piston tops, eats away at head gasket ring and valves.
It also generate more heat so once it starts its hard to stop until cylinder cools down a bit.
EGR system reduces heat in a cylinder, thats to prevent spike in NOX toxic gases, but it also helps prevent pinging because of lower heat in cylinders.


Spark timing is not fixed, that's why distributors had centrifugal advance and vacuum advance BOTH

When a spark plug fires the "explosion" of the air fuel mix is not instant, as the flame spreads it takes a few millisecond for full "explosive force"

For best engine performance you want full "explosive force" to happen about 10deg After TDC, this gives the best leverage for piston/connecting rod to push down on the crank adding best power.
A known air/fuel mix will burn at a known rate, so that can be set, but a Richer mix burns faster, so has to be accounted for.
And piston speed is also a factor
At 500rpms it takes piston XXXX amount of time to move from 20deg BTDC to 20deg ATDC
At 1,000rpms it takes XX amount of time so spark timing has to account for that

Centrifugal spark advance was for RPM spark timing, as RPMs increased spark was moved farther from TDC(advanced) because burn rated stayed the same but piston was moving faster from 20deg BTDC to 20deg ATDC.
Vacuum advance was for fuel mix burn rate, when you opened throttle(add more gas) vacuum would drop and spark would be less advanced for that RPM because richer fuel mix burns faster.

So thats the basic of spark timing.
Computer now does the spark timing because it knows RPMs and exact fuel mix so much better than the older way.

If knock sensor detects ping/knock then computer will advance spark timing farther Before TDC to "head off the ping" this causes full "explosive force" to occur closer to TDC so less power is added to crank.
But it beats melting the pistons


You can get a Bluetooth OBD reader and watch Knock sensor in real time to see if it is detecting knocks all the time on regular gas.
Could be there is carbon build up in the cylinders or EGR system that is causing this.
9.7:1 is pingy on regular gas after it is fully warmed up and pulling a load, but just cruising around it shouldn't be that pingy

 

Thanks Ron, awesome explanation. I'm not having any problems with my engine running on 87 octane. I will check it with a scanner to see how much the knock sensor is "detecting" like you mention. I just got curious when I read one of your other posts about 9.7:1 compressuon engines needing knock sensors to run on 87. My first thought was... "why not just spend the extra 10 cents a gallon for 89 octane so the engine runs at it's best without needing to rely on the knock sensor?" Obviously it's more economical to run 87, but for my truck which I fill with gas about once a month it's not really a major expense.

 

Thanks

Yes, run less expensive gasoline, it was designed for that, if you were going to be pulling a load on a longer trip then it would be beneficial to add some higher octane fuel for that trip.

 

I apologize for reviving such an old post, but I was doing some research on knock sensors and your posts are always so informative. But I had a question on this one. I have always heard that when the computer detects too much pinging, then the timing is retarded (delayed) and not advanced. Is that not the case? Just so I make sure I have my terminology correct, by "advancing" I mean the ignition spark happens earlier in the compression stroke (like going from 20 before TDC to 40 before TDC is advancing the spark timing).

I have a couple of code readers and both of them read the knock sensor output as some number between 0 and ~1000 (at least that is the most I have ever seen). But the Ford service manual and pretty much every discussion on knock sensors say the PID output should be a voltage.

 

Yes, reducing(retarding) the spark timing closer to TDC is more common, the computer will attempt both directions to eliminate the "noise"

The ping/knock noise is caused by multiple ignition points inside a cylinder, its why diesels sound like they do, if the air/fuel mix is not evenly distributed then several pockets ignite at the same time causing the ping/knock noise, diesel engines are built to withstand some pinging and knocking because its unavoidable in compression ignition engines
In gasoline engines the spark plug should be the only point if ignition, if there is a secondary ignition point then there is a ping or knock noise, spark timing is changed to match the secondary ignition point so there is just one ignition point

"Detonation" is the more common ping/knock, this occurs AFTER spark plug has fired, so to eliminate this type of ping/knock spark would be "retarded" a bit to match the secondary ignition, this type of ping/knock occurs because pockets of air/fuel in the cylinder didn't ignite from the spark but did ignite milliseconds later from the pressure/heat of the main ignition
Retarding spark timing also reduces HEAT in a cylinder so can also eliminate the other source of pinging and knocking

"Pre-ignition" is when the secondary ignition happens BEFORE spark plug fires, to eliminate this the spark would be "advanced" a bit to match secondary ignition, this type of ping/knock occurs when HOT spots developed in a cylinder, carbon build up, hot spark plug tip or lean fuel mix, low octane level

Pre-ignition is often what people think of when they read about pinging/knocking so "advancing" spark timing makes more sense to them

EGR system reduces HEAT in cylinders, this reduces the amount of NOX production, a toxic gas, and because it lowers the temp it lessens chances of both types of ping/knock