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Run richer? Do have any links to a site that explains this? I'm 99% sure that does not happen.
Here is my understanding. Knock is caused by a secondary ignition inside the cylinder. The primary ignition starts with the spark plug. As that flame kernel expands across the cylinder, it compresses the unburned fuel in the cylinder. In a spark plug ICE, fuel takes a several thousandths of a second to burn. It is not instantaneous. Anyway, this expansion of the flame front across the cylinder behaves somewhat like a shockwave from a bomb. It has a pushing effect. This pushing effect increases the compression of the unburned fuel along the perimeter of the cylinder. That extra compression is enough to raise the temperature of the unburned fuel above its flash point and it detonates. In effect, the unburned fuel compression ignites. What the cylinder gets are two separate “booms” inside the cylinder. One from the spark plug (desired) and another very shortly after that from compression ignition (undesired). This secondary ignition also has its own “shockwave.” When the two shockwaves collide, it massively increases the pressure inside the cylinder. The pressure spike is so great that it pushes the valves sideways in the cylinder head (ping) or worse, it pushes the piston sideways in the cylinder (knock).
By retarding the ignition timing, the ECU starts the primary ignition closer to top dead center. By starting the ignition later in the compression stroke, it pushes some that burn-time deeper into the expansion stroke. Once the piston passes top dead center and starts moving down the cylinder, cylinder volume increases and pressure rapidly decreases. That decrease in pressure prevents that secondary compression-ignition from ever happening and eliminates the engine knock.
If I recall, engineers want the fuel to fully combust before 18° after top dead center. Any fuel that still burns beyond that point/time no longer pushes down on the cylinder as it expands. The more the ECU retards timing, the more of that fuel combusts during the power stroke. That is what decreases efficiency (torque). The same amount of fuel ultimately combusts, but less of that heat expansion gets converted to work. Circling back, the ECU does not increase the fuel mixture when it retards timing. It simply starts the ignition process later in the cycle, and pushes the point in which the secondary ignition might happen into the power stroke, where it really can't happen anymore.
Also, octane is about activation energy. How much energy does it require to start burning? How much heat does it need to self-ignite? Higher octane fuel needs more energy (heat) to start burning. It resists burning more. Therefore, it can handle more “perimeter compression” from that expanding flame kernel than lower octane fuel before compression ignition begins. For every few degrees more before top dead center the ECU can ignite the fuel, that much more fuel combusts before that 18° after top dead center point. That is how the RDX makes 272hp on premium versus 252 on regular.
Anyway, if you can provide a link that supports your richer fuel mixture statement, I'd like to learn about it. Thanks.
I keep looking for hard data, and what I keep finding are anecdotal reports from owners as well as the theory that the ECU richens up the mixture with lower octane fuels to prevent LSPI and this is the cause of the dilution, but I have yet to find any solid "data" to present other than ignition timing pulls, which may or may not correlate.
Believe it or not, some people are fine with the ~250 HP it likely makes using 87.
Personally, I wish the RDX had a hybrid option, fuel economy focused, not performance. 225-250 HP, 30+ mpg. It CAN be done, Toyota does it on the Highlander Hybrid, a 3 row that gets 35+ mpg. 243 hp. Certainly, with the smaller footprint, the RDX would do real well with a hybrid.
Completely agree. I would have made another vehicle choice if I had privy to the "real world RDX mpg" and "knew gas prices would almost triple in two years" when I bought my 2021. The no hybrid or EV options from Honda/Acura is hard to swallow with fuel prices north of $4/gal (some places $7/gal) and rising.
On a day-to-day basis, using the 87 to produce 250ish HP (similar to what the same engine in the 2.0T accord makes with regular gas) is not really noticeable compared to 272hp with premium.
Last edited by Texasrdx21; 03-14-2022 at 10:56 AM.
I keep looking for hard data, and what I keep finding are anecdotal reports from owners as well as the theory that the ECU richens up the mixture with lower octane fuels to prevent LSPI and this is the cause of the dilution, but I have yet to find any solid "data" to present other than ignition timing pulls, which may or may not correlate.
Low Speed Pre-Ignition (LSPI) is a different story. That is a function of boost – high boost at low rpm (<1,500 rpm) to be specific. As a general rule, boost causes excess heat. If the turbo forces 1.8 times as much air into the cylinder, the cylinder burns 1.8 times as much fuel and creates about 1.8 times as much heat. That heat spike is fast and can make engine knock more prevalent.
To combat that short-term spike in cylinder temperatures, the ECU over injects fuel into the cylinder. Nice cool fuel – the temperature is about the same as the temp in the gas tank. That extra “cool” fuel instantly reduces the temperature of the cylinder and in turn, that is what helps to control engine knock under heavy boost. And yes, that extra fuel implies a rich fuel mixture, way above the stoichiometric ratio. You are right on that.
This always happens under heavy boost, not just LSPI. In fact, it is the primary reason turbocharged engines become gas hogs compared to NA engines when under load. To control heat under any heavy load (turbo boost), the ECU over injects fuel. If that cooling effect is not enough to control engine knock, the ECU can reduce boost. The ECU can also pull timing or any combination of the three. But pulling timing in itself does not make the fuel ratio rich.
To add, 100% of the time the engine pulls timing to control knock. Why? Because Honda bases its ignition map on 96-octane fuel. That is where ECU “wants” to be. With any octane lower than 96, the engine knocks. The ECU “listens” for that knock and pulls timing until it no longer hears it. This is an endless procedure. It changes second by second based on load, boost, temp, etc. It is not that the ECU advances the timing more on 93-octane than on 87-octane. It is that it pulls timing less on 93-octane (compared to 96-octane) than on 87-octane. The ECU is always finding that sweet spot. Even at a steady 30 mph, with no boost, near zero load, the ECU will still pull timing more on 87 than on 93. But the fuel air mixture will be the same. Most of the time, that is what's happening. Only for those brief seconds under heavy boost does the ECU over inject fuel.
Low Speed Pre-Ignition (LSPI) is a different story. That is a function of boost – high boost at low rpm (<1,500 rpm) to be specific. As a general rule, boost causes excess heat. If the turbo forces 1.8 times as much air into the cylinder, the cylinder burns 1.8 times as much fuel and creates about 1.8 times as much heat. That heat spike is fast and can make engine knock more prevalent.
To combat that short-term spike in cylinder temperatures, the ECU over injects fuel into the cylinder. Nice cool fuel – the temperature is about the same as the temp in the gas tank. That extra “cool” fuel instantly reduces the temperature of the cylinder and in turn, that is what helps to control engine knock under heavy boost. And yes, that extra fuel implies a rich fuel mixture, way above the stoichiometric ratio. You are right on that.
This always happens under heavy boost, not just LSPI. In fact, it is the primary reason turbocharged engines become gas hogs compared to NA engines when under load. To control heat under any heavy load (turbo boost), the ECU over injects fuel. If that cooling effect is not enough to control engine knock, the ECU can reduce boost. The ECU can also pull timing or any combination of the three. But pulling timing in itself does not make the fuel ratio rich.
To add, 100% of the time the engine pulls timing to control knock. Why? Because Honda bases its ignition map on 96-octane fuel. That is where ECU “wants” to be. With any octane lower than 96, the engine knocks. The ECU “listens” for that knock and pulls timing until it no longer hears it. This is an endless procedure. It changes second by second based on load, boost, temp, etc. It is not that the ECU advances the timing more on 93-octane than on 87-octane. It is that it pulls timing less on 93-octane (compared to 96-octane) than on 87-octane. The ECU is always finding that sweet spot. Even at a steady 30 mph, with no boost, near zero load, the ECU will still pull timing more on 87 than on 93. But the fuel air mixture will be the same. Most of the time, that is what's happening. Only for those brief seconds under heavy boost does the ECU over inject fuel.
This is probably why my CX5 averaged 5mpg more than my RDX while making 30# more torque and only 22hp less. Mazda uses a better system than pumping the fuel in the cylinder.
Also, are you saying the engine maybe makes 290bhp on 96 octane, arbitrarily? Do you have a source for this benchmark of 96 octane for the K20C4 in the RDX?
Also, people seem convinced that on 87 octane, it makes 252bhp. Where is the proof of that? The Accord? That isn't sufficient to convince me this is accurate. Did someone dyno a FWD RDX and discover this?
Last edited by Unobtanium; 03-14-2022 at 11:25 AM.
Also, people seem convinced that on 87 octane, it makes 252bhp. Where is the proof of that? The Accord? That isn't sufficient to convince me this is accurate. Did someone dyno a FWD RDX and discover this?
Same 2.0T engine, 10 speed transmission and in a similar Honda product. Accord may pull more HP on the dyno - as the 19" wheels may or may not be lighter than the 20" RDX Aspec wheels - rolling mass to move.
Those HP #'s from Acura are at the flywheel - NOT wheels. See the Dyno information runs below from Hondata.
As pertains to detergents in our fuel I was reading about the "Top Tier Gasoline" program. Acura is part of the Top Tier program. So is Costco by the way. Its not just a logo, they have to prove that their fuels meet th performance criteria.
Gasoline marketers agree when they sign on to Top Tier program that all their grades of gasoline meet these standards. However, premium grade gasoline may have yet higher levels of detergent additives. Typically, Top Tier gasolines will contain two to three times the amount of detergent additives currently required by the EPA. The extra additives are estimated to cost less than a cent per gallon.
I have a 2010 TL that my daughter drives, now she is taking a class on the other side of the town between 8 to 5 and needless to say the traffic is really bad. So I drove her twice a day. The trip is 28 miles one way so I drive 112 miles back and forth twice. I pretty much always use #93 in the car, but given the high gas price and I have a lot drive to do, I got #87 instead for two fill-ups, and averaged 28.7mpg (both by the car and my own calculation). I was a little surprised to see the good mpg, while mostly highway, there is indeed quite a bit traffic in at least 1/3 of the road. I switched back to 93 in the most recent fill-up, I got only 26.5mpg by the onboard computer. I only drove it for one round trip though, and Friday afternoon had less than average traffic, but the result is interesting. I will see what I get at the fill-up.
Here is a reason for 2022 PMC owners to use 91 ethanol-free, the blue pump handles at the local Maverik stations are a perfect match. And the car loves it. 91 with 10% alky is currently $5.69. The blue stuff is just 20 cents more.
What is the data (not opinions) on dilution with gas for those that use 91 vs 87? I will send oil sample soon for analysis, I am at 20% with 7k.
What makes you think that octane has anything at ALL to do with that issue, which is primarily due to an overly rich fuel mixture at startup, and short trips. Octane is all about resistance to detonation under heavy load, and that doesn't happen at startup.
What makes you think that octane has anything at ALL to do with that issue, which is primarily due to an overly rich fuel mixture at startup, and short trips. Octane is all about resistance to detonation under heavy load, and that doesn't happen at startup.
John Davies
Spokane WA
When running lower octane than recommended, it might run on more rich fuel/air mixture during cold starts, which may have more incomplete combustions. The only way is to to try 87 vs 93 through oil change and compare % of oil dilution given the same driving pattern. We had few people with high percentage of gas in their oil, now I am curious if they ran 87 or 91, or if it matters ar all.
Last edited by russianDude; 06-21-2022 at 11:55 AM.
When running lower octane than recommended, it might run on more rich fuel/air mixture during cold starts, which may have more incomplete combustions. The only way is to to try 87 vs 93 through oil change and compare % of oil dilution given the same driving pattern. We had few people with high percentage of gas in their oil, now I am curious if they ran 87 or 91, or if it matters ar all.
Believe that issue was due to using the Honda/Acura turbo engines for short drives, where it does not get up optimal running temps to burn off the rest.
Engine knock is almost nonexistent in a cold engine. A cold engine has so much natural "resistance to detonation," the ECU must run a super rich fuel-air ratio just for it run. Plus when an engine is cold, doesn’t the ECU operate under an open loop, as in a fixed set of instructions completely independent of feedback? I could be wrong, but due to open loop operation when cold, I thought the fuel ratio map and the ignition timing map are the same regardless of octane. If I’m wrong, please let me know.
Moving on, the primary cause of oil dilution is blow-by (boost) during that time a direct injected, turbocharged engine is cold and runs a super-rich fuel ratio. Any boost during that cold period pushes a lot of that extra fuel past the rings into the oil. That’s how the 99% of the gas gets in the oil. That is the root cause. As Texasrdx21 stated, if an engine is driven for a long time after it is hot, a lot of that gas in the oil will evaporate. However, if you stay off boost when the engine is cold, the engine may not even experience any meaningful oil dilution. A heavy foot (boost) on a cold engine is a bad combo for oil dilution.
03-14-2022, 09:59 AM
