The way I always understood it is the higher octane rating is a measurement of how resistant the fuel is to igniting before a spark hits it. So it will resist auto ignition from high compression.

 

That is my impression as well.

 

High Compression and heat. Takes both.

 

Heat comes with compression by default.

 

Glow plugs... compression alone is not enough

 

HEAT...this is why almost all engine dyno's run the engine temps in the low 100's when making pulls; little to no chance of pinging...or what ever the proper term is.

 

So how does a diesel engine work then? Why are there intercoolers on forced induction engines?

 

I think he was talking "w/in the context of a gasoline engine". We know that a diesel has dramatically higher compression. I'm not familiar with any gasoline engines that start from cranking, alone, with no spark.

 

That wasn't my point Tom. In post 61 and 62, posters made their understanding that higher octane was to resist preignition from "higher compression". In post 63, he said "and heat, you need both". My point was that heat automatically comes with fast compression of air - gas, diesel or air compressor. It is thermodynamics and cannot be disputed.

 

My point is you need a heat source to get ignition or pre-ignition and not just compression alone. Either a hot engine, spark, or something to warm it up.

Good luck getting gas to ignite in a gasoline engine without any other heat source, and I also don't see a gasoline engine able to run enough compression on any readily available street gas to create pre-ignition on compression alone.

Forced induction intercoolers: because you are doing work on the air and increasing its energy, this shows up as heat and pressure. The intercooler helps shed heat of the charged side producing a more dense oxygen charge. More Air can take more fuel and create a bigger bang. In all honesty, you are making my point for me. While it is cooler, the cooler denser air will create a higher dynamic cylinder pressure under boost with less heat in the combustion chamber... reducing the chance of pre-ignition occurring even at higher cylinder pressures. Per thermodynamics... which I did very well in BTW. Sans intercooler the air is less dense, less boost, less dynamic cylinder pressure, more heat, and more likely to ping while making less energy from the explosion.

In the case of a turbo, heat passes through the charger unit and contributes to the heating of the air being compressed in the compressor section of the turbo. If left uncooled, this hot air enters the engine, further increasing internal temperatures. Again more heat contributing to pre-ignition.

 

Yes Tom we are, but even a diesel won't get going without glow plugs. Even then, especially older ones, still need to get heat soak in the combustion chambers before it actually starts running well. Even with the dramatically higher compression ratio, compression alone is not enough.

 

Do you honestly believe that??? There are MILLIONS of diesel engines out there that have no glow plugs!

 

Yes I do depending on temperature. As you will see I also made a more general statement of something to get it warmed up. Try owning a diesel in certain parts of Michigan and needing to find a plug everywhere you park for more than a couple of hours to keep your engine warm. It only gets worse as you go North....

 

Kyle, He is right about that^^. Our entire fleet of snow cats fire up w/o glow plugs in sub freezing temps. Some of them have grid heaters but will still start when the grid heaters fail (which they fail a lot). Some have ether start assist bottles but those things run out and the op's never tell us....they still start up fine.

BUT that's a diesel. We're talking 20:1 compression on a very small cam = stratospheric cylinder pressures. Not really comparable.

 

The air in a High Compression engine gets Very hot, Very fast as it is compressed. Higher octane gasoline is more "stable" until it is ignited by the spark plug. It is harder to start the burn process in a high compression engine and you don't want a gasoline that ignites pre-maturely. The higher compression requires you have a good strong spark to start the burn and this is why I use a Multiple Spark Discharge type of ignition system and have been using bigger gaps on my plugs. The sparks sound like arc welders below 3000 rpm.

A diesel engine runs based on compression alone causing the auto-ignition of the fuel. The beauty of diesel is that it requires no ignition system since the compression alone causes the burning of the fuel. It uses the auto-ignition of the fuel when compressed with a 18-1 (and higher) compression ratio. The speed/rpm of a diesel is changed by the adding or subtracting of fuel, no air control like on a gasoline engine. I had a buddy who did not believe me so I removed the intake manifold off the diesel and ran it without the intake. Even without the intake the engine can be sped up or slowed down by controlling the fuel flow.

Hey Kyle, Glow Plugs are only used in cold weather to "assist" in starting and they are used on highway vehicles that need to start in cold weather like my family's diesel Passat. The Passat Glow Plugs get little use here in Virginia.
I installed a diesel (myself) in my Houseboat while in College. It had no glow plugs in it at all and did not need them. It was a Perkins 4-236 (4 cylinder, 236 Cubic inches) diesel and made 85 hp without a hair drier. It always started up in any kind of weather and got me all the way back to Northern Virginia from North Fort Myers, Florida on less than ~ $300 worth of diesel.

On a Automobile engine with high compression you want to be sure that you get gasoline that will wait until the spark fires to ignite it. It is possible to have carbon (and other matter) inside the cylinder that can glow hot enough to detonate the fuel and this can hurt the engine if left unattended. High compression makes power but with today's junk they call gasoline you need to start with the highest octane to prevent Detonation or Auto-Ignition from occurring. My exhaust system is a set of Hedman Hedders and the first 5" of each outlet pipe has the coating burned off, the higher compression requires a higher heat rated ceramic coating over a normal low compression engine.

The L88 engines used High Compression, Aluminum Cylinder heads and a moderate camshaft to make close to 560 hp from the factory. But when the L88 was built they could go to Sunoco and get 100 plus octane leaded gasoline which made it perfectly happy. Today running that setup on the highways requires a few tricks but can be done fairly economically. It has been a decade or more since I bought race fuels. When my engine is running it sounds like it is popping and spits the exhaust out, it does not sound like it a normal gasoline engine.

This explains why I have my Snow performance water/methanol injection system operating when the engine gets Hot and during the summer months. The water/methanol combination can slow down the burn inside the cylinder by adding methanol to boost the octane and cools the fire a little by adding the water. The water/methanol injection makes a High Compression engine "work" on the highways without any problems. When the system is injecting the fluids the high compression is no longer a problem. Snow performance sold their systems primarily to people with Turbo chargers or super chargers to prevent detonation. I don't have either but I do have issues with gas quality and high temperatures.
I also buy the plain old .99 cent jugs of windshield washer fluid to add to the injection tank, they are roughly 40% alcohol and 60% water in the mid eastern coast line but in colder environments they get up to 50/50 water and methanol. Fortunately there are a lot of sources of windshield washer fluid when traveling, finding High octane fuel is not always as easy.

 

Some tid bits...

IIRC, the D9 Caterpillar dozer my father used to drive when I was a kid would use an axillary gasoline engine to first, circulate coolant through the diesel engine which it would warm as it would warm up, then when the much larger diesel engine was warmed, would provide the starting power to turn the diesel engine over and get it started. This was very efficient for cold weather starting

When I was little, my dad had a yacht which had been a northern European coastguard patrol vessel. He bought it as surplus military and converted it into a yacht. The engine, a Stork Werkspoor diesel, well that was quite a story. It was massive, almost 6' tall, with a gigantic flywheel. Each cylinder had a shelf in the casting, on which you placed a blow torch. They had to be lit about half an hour before you wanted to start the engine

I've not seen it done personally but I've been told that some old (WWII) military engines would be warmed up prior to starting in any number of ways which anecdotal included the use of lit marine flares

To expand on the answer above, many old vintage diesels ran a 'hot bulb' design. There would be a section of the head where an open flame would be used to heat it, so the engine could ignite the lower grade fuel oils/diesels without the need for extremely high compression


I guess those are all referring to much larger diesel engines... I do not play with diesles much, but my old 7.3 didn't like to start cold at all... and would take a min to warm up in a Michigan winter before it would run right. Would it start, yes eventually, but you either waited on the glow plugs or had extended cranking times. Which back to my point, you had enough compression events to start to build heat. With the engine already warm, running was almost instantaneous. Extended cranking would be classified under "Something to get it warmed up"

 

This!^^^^^. Very knowledgeable and accurate post. Also points out why diesels require external vacuum pump to operate accessories.

 

It also absorbs heat in the intake tract as it vaporizes.

I agree with what you are saying, 100%... assist is a good word for it. However, I will tell you in this cold climate, giving the glow plugs time to do their thing resulted in an engine that was almost ready to go where as cranking until it would start... and it will... resulted in it being noticeably louder and down on output for the first mile or so until it got warm. Best option was to keep the block heater plugged in for any extended stop times.

 

And you still need a heat source, compression alone is not enough!

And @ctmccloskey brought up the Perkins 4-236... interesting information on starting them cold from their own manual....
In section 3 here...http://www.moteur-perkins.com/upload...s-ld-4-236.pdf

Mentions use of cold start fluid in various forms or expect about a 15 second cranking time. Either using a highly volatile starting fluid that will react without much heat or enough cycles to build heat...

Again showing, compression alone it not enough. It will not fire off on the first cycle with only the heat from the compression when cold on regular diesel fuel.

Would we like to return to discussing gas motors with compression ratios related to pump gas?

 

OK, I thought it was a given that the discussion was about a running engine. Otherwise there is no high compression or heat.