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DI is high pressure, making the fuel delivery more precise and to atomize fuel better, also because it's being introduced to the cylinder under compression, yes. So there would be LESS chance of pre-ignition. Miller cycle indicates intake valve remaining open and pushing some air-fuel mix back out the intake valve, I believe you would need a scroll-type supercharger to facilitate that, a turbo would not produce the boost necessary at lower RPM's
Also I would add the ignition and combustion cycle is different here than in Europe, I believe the flex-fuel cars here have a similar sequence to those Euro cars.
Intake valve is closed well before the fuel is injected in the cylinder so unless there is a leak at the valve no fuel/air mixture will flow thru the intake valve.
An engine designed to run on premium fuel, will not burn regular as efficiently, usually some type of dished piston is used to reduce this in cars designed to run regular in direct injection. hence more unburned fuel will be pushed out the exhaust valve into the exhaust system, this can mess with O2 sensors.
I do not agree with this. As fuel is injected in the cylinder just as when it is ignited the low octane rating should not mean the fuel does not burn completely if higher octane fuel does. Octane rating plays a major role when air/fuel mixture is compressed together but when fuel is introduced just at the end of the compression cycle the octane rating should not matter that much.
Intake valve is closed well before the fuel is injected in the cylinder so unless there is a leak at the valve no fuel/air mixture will flow thru the intake valve.
Might want to read through the "Miller Cycle" which is what I was responding too
I do not agree with this. As fuel is injected in the cylinder just as when it is ignited the low octane rating should not mean the fuel does not burn completely if higher octane fuel does. Octane rating plays a major role when air/fuel mixture is compressed together but when fuel is introduced just at the end of the compression cycle the octane rating should not matter that much.
You are entitled to your opinion
Last edited by pierrejoliat; Nov 30, 2023 at 12:26 PM.
My e350 engine is a flex fuel engine. I would think that MB would have made adjustments for e85? Maybe they didnt?
They did, your gas tank is bigger, your coil packs and spark plugs are different, all of the fuel system seals and o-rings are different and your computer is programmed to work at a wider parameter of octanes, your combustion system is also programmed more like a Euro E350
Yes, I have one, I noticed you didn't lead with "I have a flex-fuel car"
no I didn’t. My initial post was a link to a story on how to mix E85 with regular fuel to boost the octane rating from 89 or 87 to get it to 95 or 93 etc. The epa rating on my car for e85 is 20 mpg. I find that it is a wash for cost vs mpg. I just filled up with 93 for my weekly 300 mile round trip to the airport to set a baseline to see how it runs on straight 93 vs a blend of E85/regular87. But my 944 turbo is tuned for e85 and spits out 480 hp and 500ish torque but that’s at 25 psi. The head bolts are stretched and the head gasket is compromised. I need to install new head studs, a new head gasket, a new cam to pull some torque out and get a stand alone computer with harness this winter to get it sorted out. My plan is to have a tune for E85 and straight pump gas. But that E85 really helps out with boost.
no I didn’t. My initial post was a link to a story on how to mix E85 with regular fuel to boost the octane rating from 89 or 87 to get it to 95 or 93 etc. The epa rating on my car for e85 is 20 mpg. I find that it is a wash for cost vs mpg. I just filled up with 93 for my weekly 300 mile round trip to the airport to set a baseline to see how it runs on straight 93 vs a blend of E85/regular87. But my 944 turbo is tuned for e85 and spits out 480 hp and 500ish torque but that’s at 25 psi. The head bolts are stretched and the head gasket is compromised. I need to install new head studs, a new head gasket, a new cam to pull some torque out and get a stand alone computer with harness this winter to get it sorted out. My plan is to have a tune for E85 and straight pump gas. But that E85 really helps out with boost.
Wow, I had a 928 back in the '80's was a lot of fun, have you had any issues in the 944 with gaskets or seals running that much ETOH?
Wow, I had a 928 back in the '80's was a lot of fun, have you had any issues in the 944 with gaskets or seals running that much ETOH?
I bought it from a buddy of mine a couple of years ago so not sure if he took precautions but new fuel pump, new injectors to flow the extra fuel and fuel lines in the engine bay. Engine was rebuilt with dry sleeves and stroked so I assume it has precautions taken since he knew he was going to E85. Guess I’ll find out what is not compatible with E85 lol. The seals are all stock stuff and head gasket is aftermarket that gets pulled every so often anyway and he overboosted it. The head lifted before e85 could have any effect on the head gasket material.
Might want to read through the "Miller Cycle" which is what I was responding too
Yes, it talks about fuel/air mixture in the cylinder while compression cycle starts and pushes some mixture out thru the still open intake valve but with DI engines we don't have air/fuel mixture in the cylinder but just before spark ignition and all valves are closed well before that or there will be no compression and with that no ignition at all.
One thing here to realize is the type of the fuel supply in the engine, port injection vs. direct injection.
With the “old fashioned” port injection fuel octane rating is very important as the fuel/air mixture sees the high compression before the ignition spark. We don’t want the fuel/air mixture to ignite before the spark so we need high octane rating of the fuel.
DI engines are a bit different for this. Fuel is not mixed in air outside the cylinder so it is only air that sees the high compression before the spark.
Fuel is injected in the high compression cylinder just at the time of the spark or just a microsecond before that. Danger for knocking is minimal regardless of the octane rating or at least way lesser problem compared to the port injection. This all makes me wonder if I should try a tank of 87 to see how it runs in my DI engine.
Now I then did that trial and the result is there was no difference in fuel consumption between running 93 and 87 octane fuels. Now, this also raises the question if the 93-octane fuel I had actually was that, and I have wondered this many times with my E550 as it is easy to feel the octane difference and you actually see it clearly in the MPG also.
This test was driven with my 2012 S550 with the DI engine.
Below pictures show the result of todays "trial". I did not notice any kind of difference in how the car engine worked at all.
The first two pictures are with running 87-octane, the second two with 93.
The drive with 93 was totally flat I-10 going east this morning and with 87 it was hilly I-65 going north. Last (first) pillar on this graph is low as I had slow local drive until stop before taking picture.
Amazingly these show exactly the same MPG at 27.8 though with 87-octane fuel average speed was higher. I had cruise set to 79 on both cases and car went that pretty steady except at some trucks etc on road causing slow downs, but these were pretty well the same. Also the 93-octane run includes about 15 miles local drive that brings the average speed down some.
For me this shows the octane rate of the fuel has little if any effect to fuel mileage with the DI engine and I know there is a big difference with the port injected one in my E550.
.It really come down to the brake mean effective pressures that you are running and whether the knock sensor is needing to retard the timing. If the engine isn't very loaded it shouldn't matter,
I wouldnt trust the mpg on the dash. I get 17 because im running 50% ethanol, but the dash says ~24. I dont believe youll see any diff with gas anyway, its mostly about power. There is a low octane mode the car can go into, where in my car it drops timing 3.75 degrees, but youd need to log data to see if it does. I also dont know how long it stays in that mode once triggered.
Now I then did that trial and the result is there was no difference in fuel consumption between running 93 and 87 octane fuels. Now, this also raises the question if the 93-octane fuel I had actually was that, and I have wondered this many times with my E550 as it is easy to feel the octane difference and you actually see it clearly in the MPG also.
This test was driven with my 2012 S550 with the DI engine.
Below pictures show the result of todays "trial". I did not notice any kind of difference in how the car engine worked at all.
The first two pictures are with running 87-octane, the second two with 93.
The drive with 93 was totally flat I-10 going east this morning and with 87 it was hilly I-65 going north. Last (first) pillar on this graph is low as I had slow local drive until stop before taking picture.
Amazingly these show exactly the same MPG at 27.8 though with 87-octane fuel average speed was higher. I had cruise set to 79 on both cases and car went that pretty steady except at some trucks etc on road causing slow downs, but these were pretty well the same. Also the 93-octane run includes about 15 miles local drive that brings the average speed down some.
For me this shows the octane rate of the fuel has little if any effect to fuel mileage with the DI engine and I know there is a big difference with the port injected one in my E550.
.It really come down to the brake mean effective pressures that you are running and whether the knock sensor is needing to retard the timing. If the engine isn't very loaded it shouldn't matter,
2016 E350 4Matic wagon, 2019 Ford Expedition, 2019 Chevy Bolt EV
Originally Posted by pierrejoliat
Now, fill your car with 87, and accelerate full pedal on a freeway entrance ramp, Listen.
and do this on a hot day, at relatively low altitudes. I had a Jetta GLI that would crackle like a popcorn popper on low octane gas at heavy throttle, then retard and you could feel the power just go away.
Now, fill your car with 87, and accelerate full pedal on a freeway entrance ramp, Listen.
I did do some quite aggressive pedals today entering the freeway and I did not notice anything wrong. Car felt strong as always. I have never floored the pedal on this car. It has sometimes spun the wheels on wet road without full gas and it cuts the power from slip control.
and do this on a hot day, at relatively low altitudes. I had a Jetta GLI that would crackle like a popcorn popper on low octane gas at heavy throttle, then retard and you could feel the power just go away.
I wouldnt trust the mpg on the dash. I get 17 because im running 50% ethanol, but the dash says ~24. I dont believe youll see any diff with gas anyway, its mostly about power. There is a low octane mode the car can go into, where in my car it drops timing 3.75 degrees, but youd need to log data to see if it does. I also dont know how long it stays in that mode once triggered.
The car always shows higher MPG than what it actually is. Today I finished the 87-octane tank of gas and at the end it said 27.2 MPG. This includes all local slow traffic with stops at lights etc. I drove total of 491 miles and it took 20 gallons even to fill it up. This gives 24.55 actual MPG. This is quite normal for this car running with 93-octane fuel too.
I filled it back up with 93 today and will see if it makes any difference, i.e., improves MPG but I doubt.
I did do some quite aggressive pedals today entering the freeway and I did not notice anything wrong. Car felt strong as always. I have never floored the pedal on this car. It has sometimes spun the wheels on wet road without full gas and it cuts the power from slip control.
If you use 87 octane in a car requiring 91, you will lose 10% efficiency, fact. if you use 87 octane in a car requiring 91, you will lose 10% horsepower, fact. if you use 87 octane in a car requiring 91, you will have extra carbon build-up and may damage your O2 sensors, fact. Fuel pre-detonation can cause serious engine damage,fact. Your car, your gamble
2016 E350 4Matic wagon, 2019 Ford Expedition, 2019 Chevy Bolt EV
Originally Posted by Arrie
And did this Jetta also have a DI engine?
nah, it was a 1989 1.8L 4 cyl CIS-E injection DOHC/16V. It had knock sensors, so when the gas was too low octane, it would retard at the expense of all the Fahrfurgnugen.
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