Don Terrill’s Speed-Talk

I have written a technical paper on the use of NOx gas measurement vis-a-vis performance tuning. NOx generally is not considered to be an 'important' gas to measure during the tuning process, but it actually is a good indicator of flame temperature early in the combustion process - so it increases markedly as you approach detonation.

Seeing this effect, I think it may be somewhat overlooked as a tuning and timing guide - but it is a simple and unintrusive way to look at combustion temperatures (and therefore, pressures) as the combustion process takes place.

However, I don't know if many of you have actually had experience using this gas that way - so I am asking for input here.

Do you think it has merit - and has it been applied to real engines in real circumstances? I know that some pretty sophisticated tuners have used it - but don't really know what the general consensus is here.

I think that in theory it has merit - but am looking for some real-world input here.

I am not versed in NOx tuning and would be interested in knowing how NOx testing can be applied to the tuning process.

I do not see any prices on the gas analyzers on your website. I would like to know what your prices are.

Welcome to the forum!

Hi Robert,
I’m remotely involved with NOx control and measurement with natural
gas fired turbine generators. Basically what we do is lower combustion
temperatures by leaning the air/fuel mixture to a point just before flame
out, that’s where the HC and NOx levels are the lowest.

I will be attempting to do something similar with and engine; extending
the lean burn misfire and detonation limits with controversial engine
modifications then leaning the mixture to the new found limits to control
NOx as opposed to the current use of the catalytic converter. I realize
running this lean is not considered performance tuning.

I’m interested in your paper and do believe your theory has merit, is
it something you can share?

There was mention in a recent thread re 4 & 5 gas analysers Vs. WBEGOs about monitoring NOx levels as a detonation indicator...

I have been watching the level of NOx in the exhaust as a indicator of over advanced ignition timing with interesting results. I can "see" if the level of NOx begin to increase as the ignition timing is increased before i actually hear a ping. I am also using the NOx reading to tune the amount of advance from the vacuum advance (if used) on a performance street engine, in most cases on a motor with a properly tuned mechanical advance curve i use 10 to 12 degrees of additional advance from the vacuum advance.

The level of acceptable NOx is the tricky part, i see 500 -700ppm as acceptable depending on the engine package and the fuel being used but i would like to hear if other people are using NOx as a tuning aid.

PS, i always confirm the air/fuel mixture curves are correct before using the NOx reading as a indictor of over advanced timing.

Thanks Henry @ oles carb

Cammer wrote:I am not versed in NOx tuning and would be interested in knowing how NOx testing can be applied to the tuning process.

I do not see any prices on the gas analyzers on your website. I would like to know what your prices are.

Welcome to the forum!

Thanks for the welcome. Very few folks are 'versed' in NOx for performance tuning - so that is why I am looking for them. Basically, this is a concept that came to me after the review of NOx generation in SAE papers and comments by performance tuners. Perhaps this has been one area overlooked - that is my thought.

I will post an edited form of the TSN (I tried posting the full thing as an article, but it was flagged as too big - so I will have to edit some things out to get it to fit here) so you can see how this gas can be used.

Regarding MSRP prices of gas analyzers - 2-Gas is $2,550.00, 4-gas is $3,295.00, and 5-Gas (the one with NOx) is $4,250.00. These are undiscounted prices, of course - so the street prices are somewhat less.

rschrader,

Thanks for the prices.

I am sure you will be an asset to this forum.

_________________________

automotive breath wrote:Hi Robert,
I’m remotely involved with NOx control and measurement with natural
gas fired turbine generators. Basically what we do is lower combustion
temperatures by leaning the air/fuel mixture to a point just before flame
out, that’s where the HC and NOx levels are the lowest.

Now this is an interesting concept - lean out the mixture (put more air in the combustion gas) to lower the flame temperature to reduce NOx. Remember, though, that this a case of continuous combustion, so I think you can go quite a bit leaner than you can on a converntional internal combustion engine.

It goes against conventional practice for reciprocating engines - but the principle is the same - add a fill gas to lower flame temperature. But - what is a comparison of combustion pressures between the two types of engines?

automotive breath wrote:I will be attempting to do something similar with and engine; extending
the lean burn misfire and detonation limits with controversial engine
modifications then leaning the mixture to the new found limits to control
NOx as opposed to the current use of the catalytic converter. I realize
running this lean is not considered performance tuning.

I’m interested in your paper and do believe your theory has merit, is
it something you can share?

Well, perhaps you should read what I will post first, but you can always drop me a line and I will email you the full text of the White Paper.

Sounds like you have an interesting spin on a lean-burn engine here.

olescarb wrote:I have been watching the level of NOx in the exhaust as a indicator of over advanced ignition timing with interesting results. I can "see" if the level of NOx begin to increase as the ignition timing is increased before i actually hear a ping. - - -

Thanks Henry @ oles carb

This is exactly what I am talking about - using NOx as an indicator of combustion flame temperature/pressure. Pretty interesting idea, I think - and you are right, Henry, you will have to empirically decide just what 'good' NOx concentration means by engine type, etc. - but the idea has merit as a non-intrusive window into the combustion process.

White Paper No. 25 Rev. 070503
Using NOx as a precursor of Detonation – its use in Performance Tuning.
Overview of NOx Generation and Measurement:
NOx is formed during the early part of the combustion process, where there are high enough temperatures to cause nitrogen (80% of air) to combine with the oxygen in the intake charge. It is made up almost entirely of NO (Nitric Oxide) – which is a gas measured in the Model 900503 exhaust gas analyzer. In general, it has been viewed as one of the smog emission gases that is controlled by a variety of methods (including EGR and a three way CAT) and measured by tailpipe gas testing under loaded-mode conditions.
It has not been viewed as an ‘important’ gas for performance tuning applications.
Lately, though, it has come into some favor with performance tuners as a precursor-to-detonation gas. This is because its generation is directly a function of early combustion flame temperature – during the time when there is a good deal of oxygen present to combine with the nitrogen of the intake charge.
Flame Temperature and Detonation:
Detonation occurs due to early combustion initiation. If the combustion process begins too early in the compression stroke, the chemical process releases heat at an ever increasing rate, due to the fact that the combustion chemical process itself increases markedly with temperature. As this process feeds back on itself, the combustion process can run away, resulting in detonation – ranging from loss of power, to pinging, to outright engine damage. The high pressures created by the early combustion process can be interpreted at high flame temperatures – as the pressure of the combustion gases varies directly as their temperature. Thus, flame temperature and detonation are very closely linked. As the flame temperature increases in the combustion chamber, it will naturally lead to detonation – and if it could be measured, it would yield valuable information regarding the detonation margin available in the operating engine.
Difficulties in the measurement of flame temperature:
Flame temperature in the combustion process has been notoriously difficult to measure, due to the speed at which it develops, the high temperatures it achieves, and the low mass that hot gases involve. In addition, the process of instrumenting the combustion chamber is difficult, involves modifications to the combustion chamber itself, and causes the combustion process itself to change as a result. Most testing where flame temperature has been of primary importance have used a pressure sensor instead of attempts at direct temperature measurement – with mixed results – as the natural compression pressure has to be first subtracted from the non-combustion compression pressure waveform. In addition, the dynamics of gas flow inside the combustion chamber during actual combustions often make the pressure reading vary with time and position of the sensor itself. Because of this, measurements of flame temperature have been expensive and difficult to achieve, and have often yielded uncertain results.

Use of NOx as a method to gage flame temperature:
It is well known that the NOx generated by an engine is the combined effect of flame temperature and oxygen availability. At combustion initiation, an engine without EGR has combustion gas mix that generally contains a known and constant mix of nitrogen and oxygen (a 10% variation in AFR effects the ratio of nitrogen to oxygen in the combustion charge by less than 0.5%). So – for performance tuned engines, the level of NOx generation can be used to assess the average flame temperature during the initial phases of the combustion process – and therefore can be used to verify ignition timing and detonation margin. As performance engines can be unique in design, and highly modified from production engines (different cams, pistons, porting, carburetion or fuel injection, stroke, bore, etc) a process which is relatively simple to use and does not require engine modifications to implement is highly desirable.
Since NOx gas can be relatively easily measured and is a non-intrusive process, it has merit as an early indicator of flame temperature, and therefore combustion pressure, and therefore detonation margin.
NOx measurement in practice – use it to verify ignition timing and other effects:
NOx measurement is essentially engine independent. It can be viewed as equivalent to a tracer gas being placed in the combustion chamber of a running engine – and then extracted and measured later. It can be done on an individual cylinder basis or combined engine output with ease – and is only dependant on the location of the gas sampling location.

In essence, as NOx gas generation on a particular engine will vary primarily with ignition timing, so it can be used to verify ignition timing and warn of impending detonation of any engine under the real operating conditions. Further, as it also varies with intrinsic combustion rate – which is effected by AFR as well, it can be used as a general guideline of the quality of the combustion process – and can even be a good indicator of issues regarding valve timing vs compression ratio, etc.

Remember that it is a neutral and non-intrusive witness of the combustion process – so the level of NOx generated in a performance engine is directly related to the combustion flame temperatures and pressures. Thus, it can be used to optimize a particular engine, guard against conditions which may produce detonation, and validate that the combustion temperatures and pressures are reasonable for a certain configuration.

Remember also, however, that it is an average indication of combustion pressures, not an absolute real-time measurement. If high combustion temperatures and pressures occur late in the combustion process, there may not be enough Oxygen to create a high level of NOx. This relatively unlikely event may cause two engines – one with a substantially lower combustion flame temperature early in the combustion process, but much higher later – to produce the same concentration of NOx as one which produces a lower combustion flame temperature earlier in the process.

It is wise to gain some experience with NOx measurement on the types of engines you experience before general conclusions are drawn regarding what level of NOx is ‘good’ or ‘bad’.

Consider NOx measurement to be just one more valuable tool in the performance tuners gas-analysis toolbox.

Hello Bob; I use Nox all the time as a tuning tool. It is just the best thing to have. When you assess the NOx in conjunction with the other gasses you can work out if the engine is lean or overtimed or undervaporized. I generally try to keep the NOx below 600 to 700 ppm but its not a rule. I mainly do performance engines and the NOx is the best gas for performance tuning. It allows insight into the fuel condition at spark time. When the NOx readings are combined with the plug readings it confirms if its vaporization or timing that is causing the NOx. NOx can be produced in a hot environment or a cold one. Mostly NOx is a too hot indicator but it can also be a too cold indicator. What is important to me is if the NOx stabilizes and doesn't continue to rise as the engine is on load for a while. Stable NOx levels is good to get to first up in a tune because it means the engine is thermally stable. If I cant get it stable then I cant power tune it as the engine will run rough and have irregular performance.
Nox reduces the available oxygen for correct combustion too.
I find that engines that make more power by tuning to high NOx levels are fundamentally wrong in some design regard and they never perform to their full expected potential.

Shrinker wrote:Hello Bob; I use Nox all the time as a tuning tool. It is just the best thing to have. When you assess the NOx in conjunction with the other gasses you can work out if the engine is lean or overtimed or undervaporized. I generally try to keep the NOx below 600 to 700 ppm but its not a rule. I mainly do performance engines and the NOx is the best gas for performance tuning. It allows insight into the fuel condition at spark time. When the NOx readings are combined with the plug readings it confirms if its vaporization or timing that is causing the NOx. NOx can be produced in a hot environment or a cold one. Mostly NOx is a too hot indicator but it can also be a too cold indicator. What is important to me is if the NOx stabilizes and doesn't continue to rise as the engine is on load for a while. Stable NOx levels is good to get to first up in a tune because it means the engine is thermally stable. If I cant get it stable then I cant power tune it as the engine will run rough and have irregular performance.
Nox reduces the available oxygen for correct combustion too.

Not much. 1000 ppm NOx means 0.05% O2 is tied up in NOx. Pretty trivial effect, really.

Shrinker wrote:I find that engines that make more power by tuning to high NOx levels are fundamentally wrong in some design regard and they never perform to their full expected potential.

This post overall is exactly the information I was looking for - and particularly the last statement. Good to see that you have correlated NOx to engine performance so completely. I support what you say, and agree wholeheartedly with your last statement. If you are achieving power at the expense of high combustion chamber temperatures and pressures - there is generally more to be gotten from the engine. This, of course, means 'abnormally high' for a particular engine type. Far be it from me to step on the toes of the superchargers, etc. - and each class of engine will have its own characteristics.

Still, I think that NOx has been largely overlooked in performance tuning, and should not be.

Thanks for the excellent post.

Best Regards,

Hello Bob; NOx measured as ppm is a fourth order as percentage so 1000ppm is 0.1% when you divide 0.1 by 20.8% oxygen its a oxygen loss of 0.0048%. Now I don't know what influence this would have on the final power output from the cylinder but if you took it as a loss of that percentage on a 110 HP cylinder then its 0.52 hp loss. on 8 pots thats 4.2 hp. I am not saying that that is how it goes I don't know that atmospheric data. Even when you go to 4000ft relative the O2 concentration doesn't change, well not here in Australia anyway.
Most of the NOx is made around the flame kernel on high performance engines and the reason is under vaporization. The race fuels take a lot more energy to vaporize and most engines are under filled and under compressed for reasons of safety. In order to control the fuel burn when the engine is maxing out the fuel energy absorption you have to have lots of expensive sophisticated equipment on board and have done $1,000,000 worth of research. To do it properly is beyond any body I know and I suppose one would have to be on a leading edge engine manufacturer like Ferrari or others like that etc.
So what people like us see is under powered engines that use the fuel in a safe easy zone.
What happens at spark time is the flame kernel is initiated in an under vaporized environment and this is similar to being lean. So the flame kernel makes NOx. The NOx produced is measured at the tail pipe as an average of all the burns. So the actual concentration of the NOx in the flame kernel is very high. This affects the development of the flame kernel immensely. It also affects the burn around the edge of the flame kernel as it goes through the second stage of CO to CO2 conversion. In practice the engines loose much more power than 4 hp they actually loose about 20 hp easily. The power change is not only from the NOx as the NOx is reduced by retarding timing so that moves the most effective pressure point etc.
Its just about impossible to predict the power effects of NOx because to fix it you have to improve mixture distribution and gasification levels and timing and arc locations and all sorts of factors that you should be working on anyway. What NOx does is it indicates the problems of vaporization and arc location and timing better than just doing pulls on the dyno and trying random ideas. NOx measurement speeds up the dyno tuning process and shows improvements straight away. I know I am going in the right direction when the NOx goes down and the power doesn't change. That seems to be the first level to achieve. When I get there I am happy and I keep down that road and eventually the engine runs so much smoother and it makes better power and response.
Dyno's don't show the feel of the engine, I recently tuned my sons car and I gained 4 hp everywhere across the rev range but he said the car is so much better on the road and its faster. Not just a little bit but a lot. Humans are better at analyzing some stuff than machines etc.
NOx reduction tuning when its not done at the expense of power is very beneficial to the vibration reduction and smoothness of traction delivery to the tires. When I tune an engine that responds by being smoother and not necessarily more powerful it always goes faster on the track or the water.
As an example when I tune race boats they go faster MPH at less engine revs. The smoother running engine transmits power more efficiently to the water. NOx reduction definitely reduces what one may perceive to be vibration, Its actually power pulsing.

Would it be possible for you to e-mail me the paper you have written to Shrinker@smartcarby.com

Shrinker wrote:Hello Bob; NOx measured as ppm is a fourth order as percentage so 1000ppm is 0.1% when you divide 0.1 by 20.8% oxygen its a oxygen loss of 0.0048%.

Math problem. NO contains one atom of oxygen. O2 in the air contains 2 atoms. So, 1000 ppm NOx is tying up 0.05% O2.

0.05 divided by 20.6 (water vapor dilutes air by about 1.5%) means a relative loss of 0.24% of the available oxygen. That means that 99.76% of the naturally available oxygen is still available for combustion.

That is what I was referring to - not that NOx generation does not show other ways that power is lost - only that tying up oxygen is not the main thing.

And I am including early flame effects here too, by the way. NOx is generated in larger physical amounts later on in the combustion process where there is physically a larger flame front, and temperatures are higher. At the beginning, sure - there is a lot of oxygen present, but the flame and physical temperatures are low. As combustion proceeds, it gets to a point where it can get pretty high while there is still a good deal of oxygen available to react with the nitrogen in the charge.

Shrinker wrote:What NOx does is it indicates the problems of vaporization and arc location and timing better than just doing pulls on the dyno and trying random ideas. NOx measurement speeds up the dyno tuning process and shows improvements straight away. I know I am going in the right direction when the NOx goes down and the power doesn't change. That seems to be the first level to achieve. When I get there I am happy and I keep down that road and eventually the engine runs so much smoother and it makes better power and response.

This is what I have been thinking. NOx is an overall 'quality' indicator - expecially when combined with dyno power. It is probably wise to trade off dyno power against NOx generation, as NOx generation indicates the early combustion process flame temperatures, while dyno power indicates the resulting torque at the operating RPM. The name of the game should be to produce the highest possible power while maintaining control of the combustion process - which is what the NOx is indicating.

NOx should show you when you are approaching a danger zone in the combustion process.

Anyway, that is what I have been thinking. Good to have empirical verification from the field, though. Often times theory doesn't quite match reality - and it is not smart to throw out reality, leaving you only one choice.

That is why I asked.

Thanks for your informative post.

Hello Bob; Yes it may seem a math problem however NOx is not just NO , it can be any number of Os; up to 4 I think it is with one N, so I think it might be reasonable to raise the O2 reduction level a little bit. And I agree its not the primary concern as to power reduction but it needs to be mentioned. I don't know if there is any way to tell what concentration levels there are of the different combinations of NOx.
What definitely happens on a dyno is when you hold the engine on maximum power at any rev point and it drops the power over time the NOx climbs. If the engine is in a proper state of tune it can hold power with no drop and the NOx stays low and stable. If the engine makes more power by producing high unstable NOx levels then I look at the application of the engine, if its long loads like road or boat racing then power has to be compromised to make the engine live.
It depends on the engine of course but I might suggest that NOx readings below 700ppm can remain stable on sustained high loads. 300ppm or less certainly can. I find high power level engines at WOT tend to drop power on a stable rpm load after 5 seconds or so if the NOx is above this figure.
Different fuels react in different ways to NOx output as some fuels contain Nitrogen. Those fuels make lots of NO.
Combining NOx with the other gas readings in the thought process of how to analyze and tune an engine is very beneficial. Regards Shrinker