Compression Ratio

[Raspy]

I love Witte generators.

Back in the mid 70's I was looking for some obscure parts and walked into a shop that serviced Wittes in Reno. They had a large Nevada map on the wall with thumbtacks all over it. Each one marked a spot where a Witte was on duty.

I stared at that map, talked to the guy about these magical engines that run on and on, and fell in love with Witte generators. I still want one. Every so often I see one somewhere and drool all over it.

Around that time I worked at a ranch in Lovelock Nevada. There was GMC (Detroit) 2 cycle 1-71 generator there that someone had taken apart the injection system on, years before I arrived. Many years later I went back to try to get it. It's still there as far as I know and has now not run for about 50 years.

Even earlier, in 1970, I was a lighthouse keeper. We often had power failures and had to start the backup generator to keep the light on. We had an ancient Buda genny with just enough compression to start if you were really persistent and threw caution to the wind with the starter. In the pitch blackness of the late night, with the wind blowing at 100 MPH, the lighthouse dark with no navigational aid for ships, I would hold the starter on as the engine cranked and cranked. Finally a pop would reduce the starter load for a second. Then another. And another. Then several in a row. Finally, the engine would just hold it's own, maybe. Me standing in the darkness and this poor old thing trying to wake up. Wind howling and the building creaking. Finally it would show some life and come up to speed. A throw of the switch and light was restored. The engine room would light up and the navigational light would come on. I never insulted it with ether, but always wondered if it was going to run. Meanwhile ships were out there looking for the light from 25 miles out. I wondered if they realized their safety was dependent on a very tired old diesel and my persistence.

Back in 1999, I was in Hawaii. Lounging at a remote beach on the Big Island one day I struck up a conversation with a fellow that was trimming trees and keeping things nice near the beach. He showed me an old building that was part of a plantation from many years ago. Inside was a very nice old one lunger diesel engine and generator set. I wanted that engine badly for my place over there, but it was part of a park and he couldn't let it go. Heck, I was willing to get it started just for the fun of it.

Big, slow turning, one-lunger diesels are one of the finest forms of machine ever invented.

 

[greasemonkeyok]

Yup. Even though it is two cyl, Exhibit No. 1 is poppin Johnny.

 

[rScotty]

Mark,
iThrow it all together and the 17.5+/- compression ratio is what you normally see in larger tractors(>~50 hp). In some of the smaller tractors (<~50 hp) you'll see higher compression ratios most likely because they have to get more work out of a smaller displacement and it's inherently harder to do on a smaller scale efficiently so the compression ratio needs to be higher.

Does this answer your question?

I'd like to expand (!) on the original question and the answers for a bit. I think that the original question was on the relationship of cetane to compression ratio. I read it, the questioner was saying that if the fuel ignites at a specific compression temperature rise, then why and how could diesel engine designers justify having different compression ratios?

That's one of those VGQs.....a Very Good Question! Let me ramble on for awhile and see if anything emerges to help with the understanding.. Might as well get comfortable. This is going to require some concentration...

The problem implied is that if 17:1 would cause sufficient compression heat to ignite the fuel then how/why could a designer go to a higher ratio? After all, if the max compression ratio is higher than the compression required for ignition, how can there be any advantage? We know that as the piston comes up the compression ratio must pass through the ignition compression ratio before it gets to go higher. And we know that passing through the ignition compression will ignite the burn. So why go to a higher ratio? In fact, how can we even get to a higher ratio? Reading the engine specs, the ratios are different so it obviously happens, but what's the advantage? And how is it even possible?

The answer to that question is - as several others have stated - in in the burn time. It takes a finite amount of time to burn through a charge, and at the early stages of the burn the expansion of burning gases is slow enough that the piston can continue to rise. These are the things a designer plays with when he selects the (ever-changing) shape of the compression area and the timing of the fuel injection.

We know that the timing of the fuel injection is critical to a diesel. This is because the air is not throttled as is the case in a gasoline engine. Throttling a diesel is done only by varying only the quantity of fuel.

The question is a good one because the answer is progress. It is true that in the early days of diesel design all diesels had about the same compression ratio. That's where the questioner is reasoning correctly. It's was just as he implies, and the reason for the similarity is that the there isn't much choice when one controls the fuel injection timing mechanically. The mechanical system for controlling the fuel injection into a diesel combustion chamber is amazingly reliable but from the designer's standpoint it is fairly crude way to time and control things. That was because the injection pressures available 20 years ago were fairly low and the injection time was controlled by a mechanical cam. In the real world, there is a dollar and wear limit to how precisely that a mechanical cam can be shaped. The result of these limitations to the designer was that much of the variation in burn time had to be was controlled by the shape of the head and combustion chamber. The combustion area itself was varied geometrically and resulted in poly or multi-spherical combustion chambers. But the largest difference was in the use of direct versus indirect fuel injection. (you should read about those)

With the advent of computers the designer could now use an electrically controlled fuel injector. The mechanical fuel injection cam was now simply used to provide a positive preload of fuel into the injector. The actual fuel injection now happens at much higher pressures (and more quickly) by using a pulsed high pressure magnetic injector controlled by a computer. All of a sudden short time intervals and high fuel flow rates were both possible. The designers had a field day. The "resolution" of the timed injection pulse could be precisely defined. Multiple injections were even possible. And wear on the injector controller was no longer an issue. The first thing the designer did was to inject a tiny bit of fuel as the piston rose, which would then ignite spread burning gas to all areas of the combustion chamber - just in time for the main squirt of fluid which could occur later in the piston's rise and at higher compression because of how the little bit of burning fuel enhanced the main ignition. Turbocharging could be used to scavenge previously wasted fuel from the exhaust. The net result was that in the 1990s much higher compression ratios and temperatures were suddenly available to the designer.

Designers are still making improvements based on that radical advancement in injection type. So far, the higher compression ratios have been used mostly to make the engine more efficient, more powerful, and cleaner burning. There is still a lot of room for improvement. In fact, the improvements in engine efficiency that have been done so far are not very large when compared to the energy available in fuel. Much is still wasted in all types of internal combustion engines. There's a lot of work to be done.

Hope this helps. It was fun, and that's the real point.
rScotty

 

[mikehaugen]

I have heard somewhere that while gasoline engines run somewhere in the 15-25% efficiency range, diesels were much more efficient... more like 85%. This is a calculation based on energy available from the fuel, and these numbers were from some years ago.

 

[Raspy]

I have heard somewhere that while gasoline engines run somewhere in the 15-25% efficiency range, diesels were much more efficient... more like 85%. This is a calculation based on energy available from the fuel, and these numbers were from some years ago.

Diesels are closer to 40%. The largest engine in the world was able to achieve 50% thermal effciency.

Check it out:


World Of Mysteries: Worlds biggest single diesel engine

 

[mikehaugen]

Diesels are closer to 40%. The largest engine in the world was able to achieve 50% thermal effciency.

Check it out:


World Of Mysteries: Worlds biggest single diesel engine

WOW!... that is quite an engine. Does it come in green? How big of a rotary mower should I get?

 

[Raspy]

rScotty,

Higher compression is not something recently achieved or done for more power.

The compression has to be high enough to always be reliable with any type of combustion chamber design. Old Mercedes engines from the late '50s were 21:1. Modern direct injection engines can make so much power because they have low compression. That means they can be turboed and triple the volume of air into the cylinder. It's that volume and the corresponding fuel that makes the big power. They just need to be high enough compression, initially, to start and direct injection types start much better than pre combustion types. They start reliably at lower compression ratios.

My Dad had a Mercedes diesel in the early 60s. We lived in LA and it would not start in the mornings without glow plugs. Mild temps and sea level pressures and no way would it start without the glow plugs. My Cummins will start reliably at my home at 5,000 feet elevation and near zero temps, with no assist and only 17:1

Modern common rail injection has the advantages of being able to always get good atomization regardless of engine speed. Can produce multiple injection events for noise and pollution control. Can easily have different timing for different situations.

In mechanical injection systems the injectors achieve good atomization by having a needle and spring in the injector tip. This pin is lifted by the injection pressure and is called the "pop" point. At about 2000 PSI the pin lifts and an atomized spray occurs. The duration of this spray is determined by the mechanical injection pump and it's "throttle" setting. The pin also prevents the nozzle from dribbling when no injection is called for. To get more fuel, the event must start earlier and earlier. So timing is affected by throttle position and not so much by RPM.

This system is extremely reliable. Bosch perfected the design with a couple of variations using either a piston pump for each cylinder (Mercedes) or a rotary pump and distributor system as in the Cummins up through 1993. In 1994, Cummins went to the Mercedes type pump and stayed with it until 1998 before going to their unfortunate VP44 pump which was their first electronic design. Old Cummins from the 50s and GMC/Detriot diesels had other mechanical systems that are not as good.

I was a little concerned about the Common Rail system used on my Cummins, but it has worked flawlessly for 280,000 miles so far. And, I was able to add a bunch of power and advance the timing simply by re-programming the computer. Wonderful!

 

[Jerry/MT]

I'd like to expand (!) on the original question and the answers for a bit. I think that the original question was on the relationship of cetane to compression ratio. I read it, the questioner was saying that if the fuel ignites at a specific compression temperature rise, then why and how could diesel engine designers justify having different compression ratios?

That's one of those VGQs.....a Very Good Question! Let me ramble on for awhile and see if anything emerges to help with the understanding.. Might as well get comfortable. This is going to require some concentration...

The problem implied is that if 17:1 would cause sufficient compression heat to ignite the fuel then how/why could a designer go to a higher ratio? After all, if the max compression ratio is higher than the compression required for ignition, how can there be any advantage? We know that as the piston comes up the compression ratio must pass through the ignition compression ratio before it gets to go higher. And we know that passing through the ignition compression will ignite the burn. So why go to a higher ratio? In fact, how can we even get to a higher ratio? Reading the engine specs, the ratios are different so it obviously happens, but what's the advantage? And how is it even possible?

The answer to that question is - as several others have stated - in in the burn time. It takes a finite amount of time to burn through a charge, and at the early stages of the burn the expansion of burning gases is slow enough that the piston can continue to rise. These are the things a designer plays with when he selects the (ever-changing) shape of the compression area and the timing of the fuel injection.

We know that the timing of the fuel injection is critical to a diesel. This is because the air is not throttled as is the case in a gasoline engine. Throttling a diesel is done only by varying only the quantity of fuel.

The question is a good one because the answer is progress. It is true that in the early days of diesel design all diesels had about the same compression ratio. That's where the questioner is reasoning correctly. It's was just as he implies, and the reason for the similarity is that the there isn't much choice when one controls the fuel injection timing mechanically. The mechanical system for controlling the fuel injection into a diesel combustion chamber is amazingly reliable but from the designer's standpoint it is fairly crude way to time and control things. That was because the injection pressures available 20 years ago were fairly low and the injection time was controlled by a mechanical cam. In the real world, there is a dollar and wear limit to how precisely that a mechanical cam can be shaped. The result of these limitations to the designer was that much of the variation in burn time had to be was controlled by the shape of the head and combustion chamber. The combustion area itself was varied geometrically and resulted in poly or multi-spherical combustion chambers. But the largest difference was in the use of direct versus indirect fuel injection. (you should read about those)

With the advent of computers the designer could now use an electrically controlled fuel injector. The mechanical fuel injection cam was now simply used to provide a positive preload of fuel into the injector. The actual fuel injection now happens at much higher pressures (and more quickly) by using a pulsed high pressure magnetic injector controlled by a computer. All of a sudden short time intervals and high fuel flow rates were both possible. The designers had a field day. The "resolution" of the timed injection pulse could be precisely defined. Multiple injections were even possible. And wear on the injector controller was no longer an issue. The first thing the designer did was to inject a tiny bit of fuel as the piston rose, which would then ignite spread burning gas to all areas of the combustion chamber - just in time for the main squirt of fluid which could occur later in the piston's rise and at higher compression because of how the little bit of burning fuel enhanced the main ignition. Turbocharging could be used to scavenge previously wasted fuel from the exhaust. The net result was that in the 1990s much higher compression ratios and temperatures were suddenly available to the designer.

Designers are still making improvements based on that radical advancement in injection type. So far, the higher compression ratios have been used mostly to make the engine more efficient, more powerful, and cleaner burning. There is still a lot of room for improvement. In fact, the improvements in engine efficiency that have been done so far are not very large when compared to the energy available in fuel. Much is still wasted in all types of internal combustion engines. There's a lot of work to be done.

Hope this helps. It was fun, and that's the real point.
rScotty

The requirement to ignite the fuel places a lower limit on the minimum compression ratio of a diesel engine. Higher compression ratios increase the amount of power per unit of airflow that the engine can produce and also the thermal efficiency of the engine (Power = K{(air flow rate + fuel flow rate) X Cp x (Tpeak-T exhaust)} and thermal efficiency =f(T peak);the higher T peak the higher the thermal efficiency.

Usually the smaller displacement engines will have higher compression ratios and I believe this is to increase the power output from the a smaller displacement (get more power from a smaller package) and to overcome the inherent increase in induction pressure losses (volumetric efficiency) that increase with a reduction in scale (f(Reynold's Number)).

As you point out. the introduction of electronics to control the timing of injection and other control functions that occured in the mid '90's certainly gave the engine manufacturers more design capability but a lot of tractor engines do not have electronic controls except for the latest models that require more control varialbles for emmission control reasons. Simmilarly with diesel auto and truck engines. For example, my '07 NH TD95D has a relatively simple mechanical timing change for Tier II emmission control based on coolant temperature but the injection pump is hydromechanically controlled and that has been around for years.

Engine development has to take into account a myriad of requirements, both technology related and economics related and it's hard to just pick out a single design variable and say that it uniquely drives the engine design. That's the point I'm trying to make.

 

[Egon]

A diesel ignites the fuel by heat of compression. and the ignition point is dictated by when the fuel is injected into the cylinder. You cannot have PRE-ignition in a diesel engine. This should be obvious since there is nothing to ignite before the fuel is injected.

Add the turbo or supercharger (boost) and the compression gets explained.

Then add the emissions factor and again the compression ratio gets fiddled with.

Most of those large prime movers are two cycle and probably do not have to meet emission standards.

 

[DarkBlack]

Exact same engine, same bore, same crank stroke, same static compression #'s, same everything...
Just change the wrist pin height, and the rod length to match, now you have a different effective compression ratio, and a different torque curve from the same engine.:thumbsup: