Over square operation and Specific Range

[ever_green]

Overview
I recently read an article about over square engine operation on aircraft engines. This was perhaps one of the most interesting reads I had for engine efficiency and operation. You can find the article here:

Advantages for Aircraft Engines on High MAP and Low RPM

Going Over Square
Basically this article explains that the best cruising or general operation of the engine for best efficiency is done at highest MAP (lowest vacuum) at lowest RPM. From our experience with BSFC, this makes sense. However this article gives a new term and calculation for finding if you are operating your vehicle in an oversquare fashion:

Oversquare means having the MAP higher than RPM divided by 100. Thus a MAP of 25" and a RPM of 2300 is such a situation. In case of turbocharged engines they are almost always run oversquare, for example: The Lockheed Super Constellation runs on a MAP of 51" at take-off with 2900 engine RPM and cruised with 44" and a much lower engine (2600) RPM.

For a naturally aspirate engine this means that you basically cannot exceed ~2900 RPM at sea level if you want to stay in oversquare engine operation. This is because at sea level atmospheric pressure is roughly 29 in-hg and 2900RPM/100 = 29. So for my driving I set 2900 RPM as my redline, but to keep the limits softer and add room for error i usually set 2500-2700 as the limit. I have also programmed MAP in in-hg for my scangauge to ensure i'm always operating in an "over square" fashion. Here is a pic of setup:



To get xgauge for MAP in inches of mercury program the following code:

TXD: 07E0010B
RXF: 0441450B0000
RXD: 2808
MTH: 01BB000F0000
NAM: "Hg

Basically if you are at 2000 RPM and MAP is below 20" then you are operating in-efficiently. In turbo cars though they are almost always operating in an oversquare fashion. So this mostly applies to NA applications.

SFC
This article goes the extra mile talks about improving MPG by controlling fuel mixtures. This might not be possible on some cars, but on mine I just en-leaned mixture using an ECU flash or hack. It claims that a SI engine gives best SFC at 17:1 AFR .

Fuel air ratio
The fuel / air ratio for best power is around 1:13 and for best specific fuel consumption (SFC) it is 1:17. Peak EGT occurs at the chemically correct ratio of 1:14.7 and peak CHT at a slightly richer ratio.

i'm running 16:1 just to be safe. Along with a more aggressive drivebywire that opens the thorttle wider at low RPM, i'm always running efficiently whether i try or not (ie if the girlfriend drives the car).

As per the article operating your engine in an over square fashion has following advantages:

Advantages
Running your engine with a lower RPM/100 than MAP has a number of advantages:

less vibration, noise, heat, and wear
longer engine life as a result of this
much less noise emitted from the engine/propeller
more efficient cylinder charging and better combustion, gives lower fuel consumption if leaned properly
less power lost to friction inside the engine

Almost always for me operating below ~1800 RPM ensures over square operation. So this basically sets a 40 mph speed limit for me for cruising, which makes sense. Almost all the speed vs. mpg charts indicate ~40 mph as the sweet spot for an avg 4cyl car. I also think this is a better method than monitoring engine load vs RPM.

So if you were in doubt about getting a longer final drive gear, don't be. Anything that lets you make the same power at lower RPM by managing MAP is the way to go.



Range and Endurance


I'm not 100% if this can relate to cars, but maximum range is achieving longest distance on a tank of gas while endurance is operating engine on tank of gas for longest possible time. I think range is what matters, because what if i just let the engine idle? it will most definately endure the longest than any type of driving.




Specific Range

This is the amount of distance that can be traveled per unit of fuel consumed. Again its for aircraft purposes but it can be a useful concept for autos. The calculation formula is as follows:



I used this power table and this BSFC chart to graph this for my car:



So roughly 30 MPH gives me the best range at 18.5 miles per lbs of fuel. not taking into account gearing and road surface frictions. although the number might be wrong, the curve pretty much explains that BSFC and Speed go hand in hand for range. below 30 mph the engine is operating at too low of RPM to produce any power and pumping losses mount and overpower the drag losses of higher speeds.

source:
Maximum Range Flying depends on several Factors

[gone-ot]

Big difference: Aircraft engines have A/F-ratios controlled by pilot, while automobiles have A/F-ratios controlled by EPA/CARB regulations and ECM's.

[ever_green]

Originally Posted by Old Tele man

Big difference: Aircraft engines have A/F-ratios controlled by pilot, while automobiles have A/F-ratios controlled by EPA/CARB regulations and ECM's.

No the AFR is controlled by manufacturers and you can always use ECU flashes to change it (as I explained above).

Honda will soon be returning to lean burn business again.

[serialk11r]

If you want to "be safe" then I don't think 16:1 is what you want. The risk of running lean is that the peak combustion temperature (not EGT) is higher when you're running slightly leaner than stoichiometric, which can damage the piston. Not saying that on a naturally aspirated engine it will necessarily but 17:1 is probably more safe.

[ksa8907]

Originally Posted by serialk11r

If you want to "be safe" then I don't think 16:1 is what you want. The risk of running lean is that the peak combustion temperature (not EGT) is higher when you're running slightly leaner than stoichiometric, which can damage the piston. Not saying that on a naturally aspirated engine it will necessarily but 17:1 is probably more safe.

I was under the assumption that stoich is also peak combustion temperature. Leaner mixtures will burn cooler but because it takes longer to burn, more heat is transfered to the piston, valves, head, etc.

I don't see how a more lean mixture would be more safe?

[mechman600]

Originally Posted by ksa8907

I was under the assumption that stoich is also peak combustion temperature.

No, lean mixtures burn much hotter than stoichiometric. This can cause bad things under heavy loads, namely, holes in things. This is why at heavy load, and engine will alway go quite rich (~12:1) to keep things cool.

Lean is also awful for NOx emissions.

[ever_green]

yes that's correct. I'm monitoring my EGT using scangauge and romraider datalogger...EGT jump significantly under stoich or leaner conditions with just a slight increase in load.

for example cruising at 40 mph @1800RPM the egt is around 500-530'c . move up to 60 mph @2500 RPM where load increases from 35% to 45% and EGT jumps to +650'c. that's quite a bit of increase for just a slight increase in throttle opening and RPM, and this is at stoich operation! don't get me started on lean tempratures at mild RPMs, i have managed to max out my sensor's voltage by just flooring it at 2500 RPM with lean fueling. Therefore i'm running richer under load and leaner under cruising condition for balance of fuel economy and reliability. sure the EGT are higher but not high enough to cause damage, which is why i'm not going leaner than 16:1, plus going any leaner can cause surging. Also it's quite difficult to stay in lean conditions as it requires low engine loads, which pretty much restricts me to ~45 mph cruising speeds. However in combination with pumping friendly drivebywire and over square engine operation it nets me good results ~38 MPG which is not bad for a full-time AWD 3200 lbs sedan.

[jcp123]

Very cool! I don't have a MAP reading I can use but I love the idea.

[iveyjh]

I run lean, as much as 20-1. My avg temps EGT at manifold is 550C and have never had it over 700C. I have run that way under load for two years or more with never a problem. The only problem I can see is someone abusing their vehicle (WOT) for extended periods of time at stoich to 16-1. Higher than that and my egt is cooler.

[serialk11r]

Originally Posted by ever_green

sure the EGT are higher but not high enough to cause damage, which is why i'm not going leaner than 16:1, plus going any leaner can cause surging

If it seems like it's working fine that's great to know, I'll keep that in mind for when I eventually get some engine tuning system. What does "surging" mean?