Canceling out pumping losses of throttle governed engines

[Kevin Johnson]

Originally Posted by Christ

Contrary - due to lack of control, the fuel timing was set non-optimal from the beginning, so that at it's normal operating speed, it was being injected at TDC or as close to it as possible, to prevent this very occurrence.

That's part of the reason I keep saying that power wasn't the main point of the experiment, and was only measured as a metric of efficiency per air temp/fuel consumption.

Hmmm.

Diesel engines normally run with an excess of air beyond that which is required for complete combustion of the fuel, particularly forced induction ones.

High cylinder wall temperatures help to keep fuel off them and available for burning. Many people think complete combustion is a given in diesels. This is not the case. This is why fuel lubricity additives improve efficiency as they lubricate the walls of the cylinder. This is also why diesel sump oil is generally contaminated with fuel.

High fuel temperatures improve atomization and increase flame speed. This yields more pressure in the early part of the power stroke where it does the most good.

Increasing the air temperature continually advances the point of auto-ignition. If the engine fuel injection timing was initially tuned for maximum torque output then there should have been some advance built in, so prior to TDC. If the engine was not tuned for maximum torque output initially then the experiment would be meaningless as there would be no maximal "fixed reference point."

Hotter and hotter air would make the flame front progress quicker and a point of inflection would quickly be passed wherein increasing force would be applied counterclockwise in a clockwise running engine (to the extent that it is not quickly reached then the engine was not tuned to its peak possible performance prior to this). More fuel would be required to equal past performance since a percentage of the fuel would be working against itself. The effect of increasing intake temperature would likely be asymptotic on the point of auto-ignition until (If?) a point was reached when there was insufficient compressed air to complete combustion of the fuel. I suspect long before that the engine would have expired or possibly the supercharger. An interesting dilemma but best observed behind rugged barriers.

I think this explanation covers the observed data.

[Kevin Johnson]

Originally Posted by Christ

I don't like the idea of drawing vacuum from the valve covers, because if you get enough air moving, it could work against the oil flow dropping back into the block, causing oiling issues and changing the center of gravity of the vehicle (allbeit not by much) as a whole, in upright applications.

I'd rather draw the crankcase vacuum from two vents, one at the back end, one at the front end, both at the highest possible point that would provide the largest distance between the vents and the oil. The vacuum ports in the block should have either wet filters or screens on them to help prevent liquid oil flow, or at least help to vaporize whatever does get out, so the catch cans and separators have a better chance of doing their job effectively.

Anyone know how to get/make dual sided seals? So that you can hold oil in and still hold air out?

Contact a seal manufacturer. This has been done in racing for at least 40 years, I'd say. Probably much, much, much longer. Some people reverse the stock seals. New seals is one of the reasons why it is more practical to hold vacuum to reasonable levels. Don't forget that oil pump, too.

The next issue is pin oiling. As atmosphere is reduced the pattern of ejected oil from the mains and rods reverts to disks in 3-space. The pins are typically oiled by the scraper rings from areas of the bore that are orthogonal to these disks. Et voila -- pin oiling problems. There are simple ways to address this but it is yet another thing in an increasingly long list. Heck, why not shift the pistons bores over like in the Honda engine to increase the efficiency while you're in there.

[Christ]

I'd agree with you, if we were talking about an engine that was designed for anything more than bench testing theories.

Regardless of what you think of the results, engineers and engine builders alike both know that hotter intake air will decrease efficiency of diesel engines. The vaporized fuel likes to be surrounded with as much air as possible, in as dense a situation as possible, to ensure a complete burn.

I'm also well aware that diesel engines run "lean" as it were. They all do. If you take the total fuel consumption in a given power stroke, and compare it to the total volume of intake air at a given RPM, you can come up with a basic AFR.

In order to measure real-time AFR, you'd have to know how much fuel was being injected at any given millisecond, and compare it to the air intake at the intake stroke prior to the beginning of the power stroke you're measuring. AFR in a diesel gets increasingly richer from a starting point of nil, so on an AFR map, it would show that the further the piston has traveled, the more fuel has been burned, and the AFR is closer to stoich with each degree of crankshaft rotation until the end of the injection event.

The "Fixed reference point" of our experiment was showing the EFFICIENCY (not output specifically, but output per unit of fuel) of the engine, not the power. Keeping the injection timed so that it always occurred at the same time in relation to the piston's stroke was actually optimal for the situation, since 0 load was the requirement for the study, and therefore, load adjusted injection timing was not necessary. Once again, the only reason for measuring torque output was to approximate a variance in efficiency based on intake temp.

When we observed that there was not as much torque output with the increasing intake temp, we began to add more fuel (secondary experiment) until the torque output "came back" to where it was in the control, with the intake/fuel temps at room temp, and the injection timing held accordingly. We actually never got the same output with the warmer air, even though we scaled the fuel in increments until the engine actually stalled before we could measure it's output.

Engineers and Engine builders alike will stand by these observations. Diesels like it cold, no two ways about it.

[Christ]

You wouldn't be referring to the R18 Honda engine would you?

They offset the crank's center line in relation to the cylinder bores so that the force of combustion was pushing directly down on the cylinder and rod assembly (approximately) instead of attempting to force it at an angle. I have a D15 Honda block that I have considered doing this with, by having it milled to fit a D16 crank, but milling it with a preference to one side of the bearing journal. The problem with doing so is that I'd also have to custom build the rear main seal carrier, and the oil pump, since they're both designed to be used in standard design.

I don't think either would be too much of a problem, since the oil system could easily be converted to an external pump, with a blockoff plate and seal on the front of the crank, and the rear main seal is just placed inside a plate that could easily be redesigned and milled.

Maybe I'll try this in the next couple years, if I ever get the money to have the engine milled the way I want it.

[Kevin Johnson]

Originally Posted by Christ

I'd agree with you, if we were talking about an engine that was designed for anything more than bench testing theories.

Regardless of what you think of the results, engineers and engine builders alike both know that hotter intake air will decrease efficiency of diesel engines.

My apologies -- I had in my mind that your control of injection timing was not that accurate from this initial statement (my emphasis):

Originally Posted by question

Were you holding the fuel injection timing as a constant parameter?

Originally Posted by answer

Engine speed was constant, and the engine was just a bench motor, so I'm going to have to say yes, we were. The engine wasn't designed to rev up and down, it was designed to hold a constant speed for test purposes.

Your answer then changed to:

Contrary - due to lack of control, the fuel timing was set non-optimal from the beginning, so that at it's normal operating speed, it was being injected at TDC or as close to it as possible, to prevent this very occurrence.

In a review this would be a bit suspect, i.e. you're appearing to alter the controlled conditions of the experiment to fit your data: a certain hypothesis. I know you mentioned you did the experiment some years back. Rigorously, I think my answer also fits the data you initially presented. It is a confound you have not removed.

You also just revealed that you did not test whether heated fuel and/or a heated cylinder would affect the results of heated air being used. Nor did you investigate what would happen to efficiency if heated fuel, a heated cylinder and an over driven supercharger was used that compressed the same number of mols of heated air into the cylinder as would be found in room temperature air.

[Kevin Johnson]

Recently I was trying to help someone with an efficiency project and he proposed doing this and many other things to an engine. Too many good ideas and not enough time or money and the project died, sadly.

Originally Posted by Christ

You wouldn't be referring to the R18 Honda engine would you?

They offset the crank's center line in relation to the cylinder bores so that the force of combustion was pushing directly down on the cylinder and rod assembly (approximately) instead of attempting to force it at an angle. I have a D15 Honda block that I have considered doing this with, by having it milled to fit a D16 crank, but milling it with a preference to one side of the bearing journal. The problem with doing so is that I'd also have to custom build the rear main seal carrier, and the oil pump, since they're both designed to be used in standard design.

I don't think either would be too much of a problem, since the oil system could easily be converted to an external pump, with a blockoff plate and seal on the front of the crank, and the rear main seal is just placed inside a plate that could easily be redesigned and milled.

Maybe I'll try this in the next couple years, if I ever get the money to have the engine milled the way I want it.

[Christ]

Originally Posted by Kevin Johnson

My apologies -- I had in my mind that your control of injection timing was not that accurate from this initial statement (my emphasis):






Your answer then changed to:



In a review this would be a bit suspect, i.e. you're appearing to alter the data (controlled conditions of the experiment) to fit a certain hypothesis. I know you mentioned you did the experiment some years back. Rigorously, I think my answer also fits the data you initially presented. It is a confound you have not removed.

You also just revealed that you did not test whether heated fuel and/or a heated cylinder would affect the results of heated air being used. Nor did you investigate what would happen to efficiency if heated fuel, a heated cylinder and an over driven supercharger was used that compressed the same number of mols of heated air into the cylinder as would be found in room temperature air.

In review, you're right. I'm afraid that the information in my head might be a little skewed because of the length of time that has elapsed since the tests were done, and I can't accurately determine whether parts of my statements are memory or "fill in the blanks". (By the way, me saying "I'm gonna have to say yes." was actually just a sarcastic way of saying "I thought you'd just read that in what I already said".)

Remember, I was 10 at the time... so we probably DIDN'T test every possible situation. The emphasis was a question of heated air intake in a diesel engine of specific speed/output, and as such the test was run under those circumstances, from what I remember.

I'm still half thinking about the R18 engine, and how I could implement the offset crankshaft in another engine, and the benefits that could be gained from it. I dunno if I'll ever actually do it.

[Kevin Johnson]

Originally Posted by Christ

In review, you're right. I'm afraid that the information in my head might be a little skewed because of the length of time that has elapsed since the tests were done, and I can't accurately determine whether parts of my statements are memory or "fill in the blanks". (By the way, me saying "I'm gonna have to say yes." was actually just a sarcastic way of saying "I thought you'd just read that in what I already said".)

Remember, I was 10 at the time... so we probably DIDN'T test every possible situation. The emphasis was a question of heated air intake in a diesel engine of specific speed/output, and as such the test was run under those circumstances, from what I remember.

I'm still half thinking about the R18 engine, and how I could implement the offset crankshaft in another engine, and the benefits that could be gained from it. I dunno if I'll ever actually do it.

Ok, I looked at the first 500 of about 6000 hits on the SAE site and will paste in five abstracts below that seemed to have some relevance.

One thing that is clear is that the statement, "Engineers and Engine builders alike will stand by these observations. Diesels like it cold, no two ways about it." is not true for all diesel engines under all conditions or fuels.


The Effect of Air Charge Temperature on Performance, Ignition Delay and Exhaust Emissions of Diesel Engines Using W/O Emulsions as FuelDocument Number: 870555
Date Published: February 1987
Author(s):
E. M. Afify - North Carolina State Univ.
N. S. Korah - North Carolina State Univ.
D. W. Dickey - South West Research Institute
Abstract:
Most of the work performed on the use of water/oil emulsions in diesel engines showed that increasing the water content in the emulsified fuel was effective in reducing NO\dx and soot emissions. Unfortunately, the increase in water content in the emulsified fuel also increases the ignition delay and may cause diesel knock. One way to reduce the ignition delay is to increase the air charge temperature. In this study, the effect of increasing the air charge temperature on ignition delay, performance and exhaust emissions was investigated. The experiments were conducted on a CLR diesel engine using baseline diesel fuel #2 and stabilized macroemulsions containing 15 percent, 30 percent and 45 percent water by volume. The air charge temperature was varied from ambient to 75\mDC and 150\mDC. The performance results showed that under same operating conditions, when neat diesel fuel was used, increasing the air charge temperature caused the BSFC to increase. When emulsified fuel was used, especially with low water content, increasing the air charge temperature improved the BSFC at low loads and increased it at high loads. The ignition delay was also effectively reduced with the increase of air charge temperature. The emission results indicated that under the same operating conditions, NO\dx and soot emissions increased with the increase of air charge temperature, while CO and UHC emissions, in general, were slightly reduced at low loads and increased at high loads.


Uncooled EGR as a Means of Limiting Wall-Wetting under Early Direct Injection ConditionsDocument Number: 2009-01-0665
Date Published: April 2009
Author(s):
Michael Boot - Technische Universiteit Eindhoven
Abstract:
Collision of injected fuel spray against the cylinder liner (wall-wetting) is one of the main hurdles that must be overcome in order for early direct injection Premixed Charge Compression Ignition (EDI PCCI) combustion to become a viable alternative for conventional DI diesel combustion. Preferably, the prevention of wall-wetting should be realized in a way of selecting appropriate (most favorable) operating conditions (EGR level, intake temperature, injection timing-strategy etc.) rather than mechanical modification of an engine (combustion chamber shape, injector replacement etc.). This paper presents the effect of external uncooled EGR (different fraction) on wall-wetting issues specified by two parameters, i.e. measured smoke number (experiment) and liquid spray penetration (model). Experiments performed in a dedicated heavy-duty direct injected (HDDI) diesel engine suggest that the elevation of intake temperature caused by delivery of external uncooled exhaust gases led to significant reduction in wall wetting. This is combined with IMEP improvement. In-house spray- and ignition modeling was used to gain insight into the measured trends.


The Effect of Preheating the Inlet Air to Study the Performance and Combustion Characteristics of Diesel Engine using Ethanol Emulsion
Document Number: 2007-01-0628
Date Published: April 2007
Author(s):
Muthuswamy Pillai Ashok - Annamalai Univ.
C. G. Saravanan - Annamalai Univ.
Abstract:
The main objective of this paper is to study the performance, combustion characteristics and to control the emissions of the diesel engine using emulsified fuel by preheating the inlet air. The present work has been carried out using single cylinder, four stroke and water cooled diesel engine. In the first phase, three sets of emulsified fuels 50D: 50E (50% diesel: 50% ethanol - 100% proof), 60D: 40E, 70D: 30E have been prepared and tested. From the experiment, 50D: 50E has given the best performance result than the other emulsified fuel ratios and the diesel fuel. In the second phase, the experiment has been conducted for the diesel fuel and the best result obtained emulsified fuel ratio of 50D: 50E, by preheating the inlet air to 30ºC, 40ºC and 50ºC. From the investigation it is observed that the effect of preheating the inlet air, the emulsified fuel has given the best result than the diesel fuel. Also it has been found that there is reduction in smoke density, particulate matter and exhaust gas temperature with an increase of oxides of nitrogen (NOx) and brake thermal efficiency.


Fuel-Air Mixing and Diesel Combustion in a Rapid Compression Machine
Document Number: 880206
Date Published: February 1988
Author(s):
Eric N. Balles - Arthur D. Little, Inc.
John B. Heywood - Massachusetts Institute of Technology
Abstract:
The influence of charge motion and fuel injection characteristics on diesel combustion was studied in a rapid compression machine (RCM), a research apparatus that simulates the direct-injection diesel in- cylinder environment. An experimental data base was generated in which inlet air flow conditions (temperature, velocity, swirl level) and fuel injection pressure were independently varied. High-speed movies using both direct and shadowgraph photography were taken at selected operating conditions. Cylinder pressure data were analyzed using a one-zone heat release model to calculate ignition delay times, premixed and diffusion burning rates, and cumulative heat release profiles. The photographic analysis provided data on the liquid and vapor penetration rates, fuel-air mixing, ignition characteristics, and flame spreading rates.

Results show that charge temperature is the most significant parameter controlling the ignition delay period and it significantly affects the heat release profile in the premixed combustion phase. Temperature and pressure have little effect on the burning rates after the premixed phase which supports the theory that this latter phase is a mixing controlled process. Variations in injection rate and swirl level have a significant impact on the burning rates. Injection rate affects the entire heat release profile; swirl primarily affects the mixing controlled phase.


Effect of Fuel Temperature on the Performance and Emissions of a Common Rail Diesel Engine Operating with Rapeseed Methyl Ester (RME)
Document Number: 2009-01-1896
Date Published: June 2009
Author(s):
Rizalman Mamat - Univ. of Birmingham
Nik Rosli Abdullah - Students Online
Hongming Xu - Univ. of Birmingham
Miroslaw Wyszynski - Univ. of Birmingham
Athanasios Tsolakis - Univ. of Birmingham
Abstract:
The paper presents analysis of performance and emission characteristics of a common rail diesel engine operating with RME, with and without EGR. In both cases, the RME fuel was pre-heated in a heat exchanger to control its temperature before being pumped to the common rail. The studied parameters include the in-cylinder pressure history, rate of heat release, mass fraction burned, and exhaust emissions. The results show that when the fuel temperature increases and the engine is operated without EGR, the brake specific fuel consumption (bsfc) decreases, engine efficiency increases and NOx emission slightly decreases. However, when EGR is used while fuel temperature is increased, the bsfc and engine efficiency is independent of fuel temperature while NOx slightly increases.

[aerohead]

would the vacuum below an ascending piston be any better than a vacuum above a decending piston?

[Christ]

Originally Posted by aerohead

would the vacuum below an ascending piston be any better than a vacuum above a decending piston?

I wondered this myself once, and thought/mentally designed a vacuum pump system that would act on the exhaust, as opposed to using a supercharger on the intake.

With this system, Cam overlap would allow the intake charge to be partially pulled in by the exhaust's vacuum pump (like scavenging, but better), as well as piston downstroke.

The pump would have to be able to move enough air to create enough of a vacuum to have a force sufficient to pull the piston up in the bore faster than it would normally be moving, though, and this amount of vacuum would probably serve to pull oil up through the rings, at best.

Another setup would have the pump just moving enough capacity to completely exhaust the spent gasses, while drawing in more intake charge through the cam overlap, but not enough force of vacuum to actually act on the pistons or rings (notably, anyway). I had proposed that this would work to cancel pumping losses on the top of the cylinders, as opposed to drawing oil and such through PCV vacuums, but I never studied it any more than enough to draw up the system on a napkin and commit it to memory, so no real research was done on it. This was only a few years ago, however, so it's still open to investigation in my head, if I ever feel like it.

I do wonder about the vacuum pump's drag on the motor, though... and if canceling pumping losses would compare to the amount of power needed to run the pump efficiently so that it could work as proposed.. if there would be any actual efficiency gain, or if it would just cancel itself out.