NOx and compression

[DanielC]

This thread might go nowhere, or it might cause an interesting discussion.

In another thread, it was commented that higher compression causes an engine to produce more NOx emissions. I fully agree with that statement, when an engine is being ran at wide open throttle.

 

However I am going to suggest that at most other operating conditions, increasing the compression does not increase the NOx emissions as much. Here are my thoughts. A higher compression engine is more fuel efficient. For the same amount of gas, it produces more power. However, since higher compression engine is more efficient, it needs less throttle to produce the same amount of power as a lower compression engine. Therefore, the temperature inside the combustion chamber is about the same for both engines producing the same amount of part throttle power, and if the temperature is about the same, so is the amount of NOx produced.

The higher compression engine also is using less gas while producing that power, and because it is using less gas, it is also passing less air through the engine, and ends up polluting less air.

[metalmonkey47]

ya know, you make a lot of sense there.

[JoeinCa]

uhhh....

 

no.

 

Increased compression creates higher cylinder temps. Higher cylinder temps increase nox. You do not "use less fuel" with higher compression engines. You use a fuel with a higher resistance to combustion. or a higher octane. A higher compression engine does not use less air. compression ratio is simply a mathematical function of bdc volume vs. tdc volume. you can have an engine with 8.5:1 compression, pulling the same amount of air in per stroke as an engine with 12:1 compression.

 

 

So, I don't know what you are trying to suggest. Everyone already knows that an engine with higher compression is more efficient, and turbo charging increases the compression pressure, while also working well as an altitude compensator.

 

Anyhow...

 

Fail

[matwood]

^^ rofl..

[DanielC]

Please reread what I posted. I did not suggest that a higher compression used a different amount of air at wide open throttle. If the displacement is not changed, that will stay the same.

 

I will say that at wide open throttle, higher compression will increase the NOx.

 

What I am suggesting is that because a higher compression engine, all other things being equal, produces more power at any throttle opening. Because it produces more power, in many cases, you will use less throttle to provide the same amount of power as before. Less throttle, less air flow, less fuel.

 

Lets take an example. You are driving down the highway at a constant 55 miles per hour. Or 80 miles per hour, if you drive that way. Your engine is producing a certain amount of horsepower, to overcome wind resistance, tire drag, and other parasitic losses to maintain that speed. Because you are in one gear, to produce that amount of needed horsepower, your engine is producing a constant value of torque. Torque is just turning force. It comes from the pressure of the hot gasses on top of the piston pushing it down. If that pressure increases, your car will start to accelerate, because the amount of torque exceeds that amount that is necessary to maintain a steady speed. Because you are trying to maintain a steady speed, you reduce the throttle to reduce the cylinder pressure, to maintain a steady speed.

 

If the torque output of two almost identical engines, at part throttle, is the same, the only difference being the compression ratio, I am going to suggest that cylinder pressures inside the engines are the same. If the cylinder pressures are the same, the temperatures inside the cylinders should also be the same, therefore the two engines should be producing the same amount of NOx, at part throttle, with the higher compression engine using less fuel to produce the same amount of torque, or power.

[OpelWagenGT]

I don't have much to add but above he said that "higher compression engines don't use less gas, higher octane causes that". Well you just answered your owner question beucase the higher the compression the higher the octane gas you are SUPPOSED to use. But that's it this thread is really interesting my only question is this.

 

If the engine puts out more NOx what does it matter? Would the differnce even be that great if any?

[nismo dr]

Its causing a discussion alright, not sure how interesting. How are you going to test your hypothesis? I agree with opel, whats the point? Are you trying to build a high comp motor that passes emissions or something?

[JoeinCa]

Please reread what I posted. I did not suggest that a higher compression used a different amount of air at wide open throttle. If the displacement is not changed, that will stay the same.

 

I will say that at wide open throttle, higher compression will increase the NOx.

 

What I am suggesting is that because a higher compression engine, all other things being equal, produces more power at any throttle opening. Because it produces more power, in many cases, you will use less throttle to provide the same amount of power as before. Less throttle, less air flow, less fuel.

 

Lets take an example. You are driving down the highway at a constant 55 miles per hour. Or 80 miles per hour, if you drive that way. Your engine is producing a certain amount of horsepower, to overcome wind resistance, tire drag, and other parasitic losses to maintain that speed. Because you are in one gear, to produce that amount of needed horsepower, your engine is producing a constant value of torque. Torque is just turning force. It comes from the pressure of the hot gasses on top of the piston pushing it down. If that pressure increases, your car will start to accelerate, because the amount of torque exceeds that amount that is necessary to maintain a steady speed. Because you are trying to maintain a steady speed, you reduce the throttle to reduce the cylinder pressure, to maintain a steady speed.

 

If the torque output of two almost identical engines, at part throttle, is the same, the only difference being the compression ratio, I am going to suggest that cylinder pressures inside the engines are the same. If the cylinder pressures are the same, the temperatures inside the cylinders should also be the same, therefore the two engines should be producing the same amount of NOx, at part throttle, with the higher compression engine using less fuel to produce the same amount of torque, or power.

 

If your torque output on two engines is the same at the same rpm, then you do not have different compression ratios(in a perfect world)... An engine is just an air pump.

 

Power production requires heat, fuel, and air. More power generally require more heat, unless you make it more heat efficient(less drag, lighter valve train and crank and less cylinder friction, which is why Benz has silica coated cylinders)

 

Higher compression means just that, higher compression pressure, higher cylinder pressure, higher cylinder temperature. If you want to cool the cylinder, you can go with alcohol/water injection, But this tends to pit the cylinders and cylinder walls.

 

If you are looking to have increased power, and still pass emissions easily, then turbocharging is the easy answer. You can adjust boost pressure, timing, and fuel curves, and if you have a shop that doesn't care much about visual inspections. So, you can reduce nox emissions at will.

 

I fail to see the point. My point is simply that the logic behind your thought process is flawed.

 

Joe

[datzenmike]

Nox emissions contribute to smog, acid rain and ground level ozone... all are bad for people.

 

 

In the combustion process hydro carbon (HC) fuel is mixed with oxygen (O2) in the air and burned producing heat, water, carbon dioxide and usable pressure to do work.

 

Unfortunately air is made of only 21% oxygen and most of the rest is nitrogen (78%)

 

Generally the higher the cylinder temperatures and pressure the more efficient a motor is.

 

Generally the higher the cylinder temperatures and pressure the more nitrogen will react with O2 and produce NOx.

 

Even at idle a high compression motor will produce more NOx than a low compression one. It follows that at the same load and RPM, it will also be making more. Think of a bath tub of hot water and compare to the natural gas flame needed to heat it. The heat in one is the same as the heat needed to heat the water but one is much hotter. The heat in a high compression cylinder is more concentrated than the lower compression one. It's the excessive heat that causes oxygen to react with nitrogen.

[JoeinCa]

Datzenmike, exactly.

[DanielC]

"If your torque output on two engines is the same at the same rpm, then you do not have different compression ratios(in a perfect world)... An engine is just an air pump."

 

I am suggesting that you have two different motors, with one having higher compression. They are both running at the same RPM, and at part throttle. Lets assume, the valves, porting, and airflow specs on the head are the same, and so is the camshaft. I know, that is probably not going to happen in the real world.

Given the same airflow into each engine, and lets assume we are staying away from power enriching circuits in carburettors, both engines will be consuming the same amount of fuel. The higher compression engine will make more power, because it it more efficient.

 

Therefore, you can run the high compression motor with a bit less throttle to to match the power output of the lower compression motor.

 

So you have two motors one high compression, one lower compression, producing, 50 ft-lbs of torque.

The high compression motor will have a higher manifold vacuum, or lower Absolute manifold pressure.

Under these operating conditions, Will the high compression motor really make that much more NOx?

[datzenmike]

A good point. If they are making the same power (even though using different amounts of fuel) I would still say the higher temp higher compression motor is making more NOx... how much more... no idea.

[JoeinCa]

"If your torque output on two engines is the same at the same rpm, then you do not have different compression ratios(in a perfect world)... An engine is just an air pump."

 

I am suggesting that you have two different motors, with one having higher compression. They are both running at the same RPM, and at part throttle. Lets assume, the valves, porting, and airflow specs on the head are the same, and so is the camshaft. I know, that is probably not going to happen in the real world.

Given the same airflow into each engine, and lets assume we are staying away from power enriching circuits in carburettors, both engines will be consuming the same amount of fuel. The higher compression engine will make more power, because it it more efficient.

 

Therefore, you can run the high compression motor with a bit less throttle to to match the power output of the lower compression motor.

 

So you have two motors one high compression, one lower compression, producing, 50 ft-lbs of torque.

The high compression motor will have a higher manifold vacuum, or lower Absolute manifold pressure.

Under these operating conditions, Will the high compression motor really make that much more NOx?

 

The problem is that in order to avoid engine misfires, pinging, detonation, all of that neat stuff on a higher compression engine, you have to have different tuning. That tuning includes different timing curves, different fuel curves. Even at part throttle low load cruising speeds, you will be using more fuel at the same rpm, because you will still be producing more heat(power), and higher cylinder temps, thereby, higher nox. This is a physics, thermodynamic, and chemistry thing, these rules don't change.

 

If you are concerned about emissions, get a late model engine with all of the emission controls. The lsx series of engine produce great horsepower, and are more thermally efficient than engines designed in the past.

 

The move towards direct injection gasoline engines is nice, as you can more finely control fuel timing. So, I would wait a year or two, and see how reliable the direct injection chevy v6 is. I would like to put one of those in my Z, 300 hp, excellent efficiency... near 30 mpg in the new camaro is pretty impressive.

 

My point is that changing compression ratios is not the real answer to nox emissions control. Cylinder cooling, and friction reduction will yield increased efficiency, allowing increased compression pressure, whilst achieving lower emissions, and using less fuel.

 

Joe

[bonvo]

all this discussion about NOx are you in california? do you have to pass smog? if so throw a car on it it helps alot im even considering putting a high flow cat on my car just so it runs cleaner i dont need one but it would as i said be cleaner and act as a resonator anyway if you dont have to smog the car why is this a discussion? if you do and need help passing smog let me know ive played the smog game and know how to make a car pass :P

[DanielC]

I am in Oregon, check under my name in my posts.

I do not have to pass any smog test with my Datsuns, even though it is currently not running.

I am sometimes wondering, what if we were to abandon all smog laws, and focus on increasing the miles per gallon a vehicle gets, it it would not actually end up decreasing the pollution our cars put out, and end up more economical to operate as well

[OpelWagenGT]

I can tell you this; I drove a '73 Opel GT with a 1.9L that my Dad and I bored out .50 over, put in high compression pistons, isky racing cam, weber down draft, and ran 92 octane fuel. That car got great fuel economy but at high RPMs (anything over 3,000) the engine sounded very angry and you could feel the higher compression. On the other hand I drive my 1200 now (which I know is also worn out and lower displacement) that I can easily take up to 5,000 even after the engine sitting for who knows how long. As much as I liked the power at the low end I felt bad pushing the engine too far.

[hang_510]

i no longer produce NOX

 

 

just get rid of the X and replace it w/an S.

[OpelWagenGT]

i no longer produce NOX

 

 

just get rid of the X and replace it w/an S.

 

Damn! that would be one scary scooter ride! :blink:

[datsunaholic]

I am in Oregon, check under my name in my posts.

I do not have to pass any smog test with my Datsuns, even though it is currently not running.

I am sometimes wondering, what if we were to abandon all smog laws, and focus on increasing the miles per gallon a vehicle gets, it it would not actually end up decreasing the pollution our cars put out, and end up more economical to operate as well

 

The bad part about NOx emissions is they increase when fuel efficiency increases for a given displacement. Engines putting out a lot of Carbon Monoxide and Hydrocarbons generally aren't running very efficient, but the NOx numbers are quite low. To get good CO and HC numbers you need complete combustion, which happens with generally leaner conditions, but the combustion temperatures go up and that causes NOx emissions.

 

There's no viable way to reduce the Nitrogen going in- it's atmospheric. You'd have to run a completely closed system, but I don't think folks would want to be running massive oxygen tanks to supply Nitrogen-free mix.

 

The only way to truly reduce ALL pollutants is to stop burning fossil fuels. The next best way is to run smaller displacement engines. Less displacement = less air in, less fuel in, which means less volume OUT. You need lighter cars, but that means giving up such things as power windows, A/C, heated seats, and the required 3 cupholders per seat that most new cars seem to come standard with.

 

Sadly, emissions standards are rated on percentage of pollutants, not total volume. You can make a V-10 monster a ULEV, but the volume of pollutants is still greater than what comes out a a 1L.