Archive for the ‘Engines’ Category

Reacting to Diesel: The Chemistry of Diesel Combusion.

August 8, 2013

It’s been a LONG time since I’ve blogged.  Thanks to Bob for the push on getting more information up here.  I spend a lot of time answering questions on Quora.com and I’m pretty proud of some answers, so I’m including them here.

This is a response to a question about the chemistry behind diesel combustion.  There are entire university programs, companies, company divisions and books all trying to figure it out, and there’s not any single answer.  if you want more gory details I’d suggest you spend some time reading through the technology guide on Diesel Emissions Online(http://diselnet.com).  Especially this article: What Are Diesel Emissions which gives a nice overview of the main components of exhaust emissions, and this chart:

Figure 1. Primary components of Diesel emissions, (credit: What Are Diesel Emissions from dieselnet.com)

Other than the main portions which are arguably benign to an extent, the small slice of pollutant emissions is the source of many global emissions regulations.

The primary polluting emissions which are of concern for Diesel engines are Oxides of Nitrogen (NOx) and Particulate Matter (PM).  Hydrocarbons (HCs) are traditionally not an issue with Diesel engines compared to gasoline engines because HC is generally created when there is a lack of oxygen to complete the combustion process.  Since Diesel engines are a lean combustion process HCs are not as much of a concern because the HCs tend to be burned later on in combustion stroke when they combine with oxygen.  HCs still exist in Diesel, but they’re traditionally the easiest emission type to deal with.

In Figure 2 two phases of injection mixing are illustrated.  The Premixed portion occurs at the beginning of injection and the Mixing Controlled portion occurs after combustion has started and we are more interested in maintaining combustion.  Both of these phases occur within milliseconds of each other each and every time combustion occurs and you hear a “clack” of a combustion event which is typical of a diesel engine.  Higher combustion pressures effectively decrease the length of time in Premixed combustion by allowing smaller injector holes which improve atomization and rely less on in cylinder temperatures to atomize the fuel before it ignites.  This decreases the rich combustion areas because the fuel is more dispersed.  It decreases the flame quench on the walls because more of the fuel is burned before it reaches the walls.  It also decreases NOx by allowing a higher percentage of EGR because fuel is dispersed more evenly so a decrease in the amount available oxygen is not as detrimental as would be with lower pressures.

Figure 2: Fuel Mixture technologies in Diesel engines. (credit: http://dieselnet.com/tech/diesel_emiform.php)

NOx is created by localized combustion temperatures in exceedence of ~1500°C.  “Localized” temperatures are a phenomenon of the diesel combustion process which are a result of fuel burning as it is injected into the cylinder.  This creates what we call a “flame front” at the interface between fuel and oxygen where there is enough fuel and oxygen mixed for combustion to occur.  The flame from is somewhat illustrated in Figure 2 in the “Initial rapid combustion” area of the Premixed combustion phase and the white/yellow flame areas of the Mixing controlled phase.  The primary technology for dealing with this issue in cylinder is Exhaust Gas Recirculation (EGR).  EGR attempts to lower these spikes in temperature by adding enough benign gasses around the flame front to absorb some temperature before it reaches a critical NOx creating point (more info here: Exhaust Gas Recirculation from dieselnet.com).  NOx which leaves the cylinder is dealt with through Selective Catalytic Reduction (SCR) but aftertreatment is outside the scope of this question.

PM is created by a lack of oxygen to complete the burning of Hydrocarbon Chains in fuel.   PM is the least understood portion of Diesel exhaust mainly because it is such a complex, unstable and varying spectrum of structures (http://dieselnet.com/tech/dpm.php).  PM is traditionally defined in two categories: Solid Fraction (SF) and Soluble Organic Fraction (SOF).  SF is primarily Carbon while the SOF fraction is primarily Hydrocarbons.  The proportion of SF to SOF is completely dependent on the combustion process and even the size of the engine.  The SF tends to form the backbone of any particle while the SOF tends to hang onto the outside of the carbon base.  SOF may be in a range of 20% to 60% of a total particle mass.  If you know about the hydrocarbon chain, this will seem familiar since the HC chain has a backbone of carbon with a bunch of hydrogen atoms surrounding.  PM could be seen as partially reacted hydrocarbon chains which have lost their ability to quickly react due to an unorganized atomic structure.  PM which leaves the cylinder is dealt with through two aftertreatment technologies: Oxidation Catalysts which deal with the SOF fraction of fuel (as well as any HC) and Diesel particulate filters which collect any remaining portions of SF and SOF to be burned at a later time through what is called regeneration.  Aftertreatment for Diesel engines is another topic all together, so we’ll have to talk more about that later.

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All About Octane

April 26, 2010

Today I’m going to de-mystify fuel octane numbers and what they really mean so that next time you’re at the pump you can make an educated choice for what fuel to put in your car.  When the manufacturer built your car they paid a bunch of test engineers, just like me, to program and tune the engine in your car for a specific octane level of fuel.  They put stickers all over the car too to tell you about which kind to use.  It’s important to pay attention to that and I’ll tell you why.  In the first video below I have 87 octane fuel in my car.  If you listen there is a distinct rasp or rattle which sounds pretty angry between 6750 and 7000 rpm.  In the second video I am using 92 octane fuel and you can’t hear the angry rasp anymore.  What you’re hearing in the first video is engine knock or auto-ignition.

Octane is a measure of how stable the fuel is under extreme pressures and temperature and how resistant it is to spontaneous combustion known as auto-ignition or “knock”.  What is knock? you say.  Knock is another term for the auto-ignition that I talked about in “The Big Bang Theory”, but in this case the ignition is happening before the engine is ready for it.  For gasoline engines knock is a Very Bad Thing.  When the fuel detonates before the engine is ready for it, it actually will increase the pressure inside the engine way beyond what it is designed for and in extreme cases will actually try to force the engine to stop spinning or to spin backwards.  Many times in performance engines which are turbocharged this engine knock will break the pistons inside the engine which will take the smile off your face really quick.

Combatting Poor gasoline Choices

OK so you ignored all of the millions of stickers all over your car that say to use premium gasoline and you used the cheap stuff.  Your car doesn’t make the noise that mine did in the video so you’re OK right?  well, the video is an extreme case and most of the time you can’t hear knock until it’s really bad.  What manufacturers do to handle your uniformed choices is add a knock sensor which can “hear” when knock is happening.  The computer will automatically adjust the ignition/spark timing to lower the heat in the engine and avoid knock and your engine won’t break. 

So what’s the problem then?  Great questions class, I’m glad your paying so much attention. By changing the ignition timing your engine is not operating at the levels of efficiency which it was designed to operate at.  Remember that engines need four things to operate, and that having each in greater quantities will add power and efficiency.  The four things are fuel, air, compression and ignition.  When the engineers who designed your engine were told that they could use 92 octane fuel to tune your car they took as much advantage of that as possible and used a higher compression ratio in your engine.  This allows them to eek the most efficiency out of your engine possible while still maintaining emissions and engine durability.  By using the lower octane fuel you’re making the computer change into a mode which the hardware was not designed for and your fuel economy will suffer. 

In the video below which is trying to sell a tuning tool you can hear what a knock sensor hears while the engine is running.  Note the distinct difference between when the engine is knocking and when it is not.

Why Higher Octane isn’t Always Better

Octane is not a measure of how clean the fuel is or how much energy is in the fuel.  If your car was designed to operate on 87 octane fuel, and the sticker under your gas door says to use unleaded fuel only then there is no reason to use the higher octane stuff.  92 octane fuel does not operate any more efficiently or burn any cleaner in your car than 87 octane will if your car was designed for 87 octane.  Higher octane fuels allow the engineers who made the engine to raise compression and take advantage of the improved fuel stability under heat and pressure, but if the engine compression ratio wasn’t designed for the higher octane then it still ignites at exactly the same time as the 87 octane stuff does.

Where do the Numbers on the Pump Actually Come From?

Octane numbers as presented at the pump are actually “Anti-Knock Index” numbers in North America.  These numbers are an average of two tests which refineries run on fuels before shipping them to gas stations.  That is why the fuel pumps say “Minimum Octane Rating (R+M)/2 Method”.

The “R” stands for the Research Octane Number (RON) method of measuring the octane number in which the fuel is placed into a single cylinder test engine and run at 600 rpm to measure how stable the fuel is.  The stability is measured by changing the compression ratio of the engine until the fuel is auto-ignited by the pressure and heat.  The compression ratio which the fuel ignited at is compared to actual octane fuel (which has a rating of 100 octane, or 100% octane) and n-heptane to determine the fuels octane number.

The “M” stands for the Motor Octane Number (MON) which uses a similar engine as the RON test but runs at 900 rpm and has preheated fuel and variable ignition timing.  The MON result will usually be a bit lower than the RON result in modern fuels.

What is Octane Actually?

Octane is a hydrocarbon with the chemical mixture:

CH3(CH2)6CH3

 It is actually only a reference fuel mixture which is used in the RON and MON testing which I described above.    It is possible to have an octane rating which is higher than 100 octane in racing gasoline fuels.

What About Octane Boosters?

Octane boosters are a bottle of fuel additive you can buy at an auto parts store.  Be aware that they are NOT a cost-effective way of boosting your octane.  They will advertise that a bottle will raise your fuel octane level by 5 points, but keep in mind that a point is considered 0.1.  So a 5 point bottle of octane booster will raise your fuel octane from 87 to 87.5.  What a rip-off.

Before emissions regulations made it important to have catalysts on cars you could buy leaded gasoline that used lead as an octane booster.  Lead is a great way to boost octane and it’s also good for the health of your engine because the lead adds a metal coating to all the insides of your engine like an extra protectant layer.  Unfortunately the lead will also coat your exhaust catalyst which makes it no-worky anymore.  Many racing fuels will use lead as an octane booster since most racing divisions do not regulate emissions and require exhaust catalysts.

Slow Down There Turbo!

April 21, 2010

If, like me, your introduction to the word “turbo” came from old school video games and you never learned otherwise then you still think that when actors hit the NOS buttons on the steering wheel of their cars in The Fast and The Furious (2001) that the buttons could alternately called “turbo” buttons.  Well, unfortunately for you and me and all of those lead astray by these video games and episodes of Knight Rider and the infamous KITT with his “Turbo Boost” feature, that understanding of turbos is incorrect.  Turbos are power boosters but they have nothing to do with the little red buttons on the steering wheel.

In this video you’re watching a Chrysler K-car which has been turbocharged. Just before the car takes off down the track you can hear a loud whine which starts low and goes to a high-pitched whine or almost whistle sound right at about 32 or 33 seconds.  That sound is the turbo charger spooling up.
 
Basics
 
Looking back at the previous article about engines needing air, fuel, compression and ignition, we can see that if we add more of these items we should get more power right?  Enter the turbocharger.  The turbocharger works by compressing air into the intake of the engine, increasing the amount of air (aka boost) and therefore the amount of fuel (remember the stoichiometric fuel ratios) and pre-compressing the air before the engine goes through the final compression for ignition.  Now with the turbocharger we have three of the important things for “the big bang” in greater quantity.
 
The turbo compresses the air with a spinning compressor wheel that is connected by a small shaft to an exhaust turbine in the exhaust stream after the engine as shown in the diagram.  Heat and exhaust velocity spin the turbine and compressor wheels to speeds well over 100,000 rpm
 
Turbo speed limit
 
The turbo speed I mentioned above is actually low compared to what many turbos are capable of.  The limit of turbo speed is determined by the strength of the material that is used for the compressor wheel and how resistant it is to flying apart and the diameter of the compressor wheel.  The velocity of the compressor wheel tips are a factor of rotational speed and the distance from the turbo shaft.  When over-speeding, the velocity of the tip of the compressor is so high that little pieces of material will actually separate from the wheel and fly into the outside wall, unbalancing the wheel.  When a compressor wheel becomes unbalanced at 230,000 rpm you can imagine the bad stuff that happens. 
 
Turbo speed is controlled by two main methods, one of the most common and widely used is a wastegate, which is essentially a hole in the exhaust manifold with a valve in it that will allow exhaust to bypass the turbine.  By bypassing the turbine the energy that was in that exhaust is not applied to the turbine to increase or maintain turbo speed.  A newer method for controlling turbo speed is with variable geometry turbos (VGT) and variable nozzle turbos (VNT).  The speed of air hitting the turbine vanes is a large factor in how efficiently the turbine collects energy from the exhaust and converts it to speed.  VGTs and VNTs work by lowering the exhaust velocity through the turbine and changing where the high-speed gas hits the turbine.  By doing this the efficiency of the turbine is reduced and less of the energy is converted into speed. 
 
Intercooling
 
When air or any type of material is compressed its temperature rises drastically.  After air is compressed by a turbo it can often be heated to temperatures over 350°F (180°C).  If you recall from my last blog about combustion I mentioned a type of ignition called auto-ignition; the increased temperature of air combined with the compression of the engine can cause this auto-ignition to happen way before the engine is ready for ignition to happen.  This is really bad.  Intercooling, or more correctly charge air cooling cools off the combustion air before it goes into the engine by passing the compressed air from the turbo through an air to air radiator in the front of the car.   Some production cars don’t cool off the air after turbo compression; these cars will usually have less benefit from the turbo than the intercooled counterparts.

Bigger is not Better

The next part about turbochargers that is important is that bigger is not always better.  In the case of the K-car the driver is using a very large turbo charger which takes a long time to spool up and be useful.  If the driver was not on a race track and didn’t have time to floor the engine and get the turbo spinning he’d never have any boost during normal driving because the large turbo has so much momentum to overcome before spinning fast enough to be useful.  Small turbos will give a car much better throttle response at low engine speeds and normal driving since there is very little momentum in smaller wheels and the speed can be increased quickly, but larger turbos will give an engine much better high engine speed power because of higher airflows through the engine.

There is also a delicate balance between the compressor size and the turbine wheel size.  If these aren’t balanced correctly then something called turbo surge can happen where the air actually stops going through the turbo and goes backwards out the compressor inlet.  It sounds like a great big bark or “chunt” noise when it happens.  The only time I’ve heard this in person is on a big-rig semi engine, so if you hear something wierd like that when a semi truck changes gears next to you on the highway you’ll know what is happening.  In the video below the wierd pulsing noise you hear at the end is the air surging and going backwards through the compressor.

The Big Bang Theory

April 20, 2010

So your car runs on this explosive fuel stuff called hydrocarbons. You may know it as gasoline, Diesel, Natural Gas, methane or biofuel etc etc. They’re all some form of the same basic chemical chain called a hydrocarbon chain. The only real difference at the basic level between each kind of fuel is the length of the chain itself. I’ll get into more detail in the chemistry section if you really want to know… but for you haven’t reached that level or nerd-hood yet I’ll cover the more basic stuff too.

In order for an Internal combustion engine to work we need four things: Fuel, Air, Compression and an Ignition Source. Fuel is important because, well, it’s the stuff that explodes to make the car go. Air is important because the oxygen in the air is what reacts with the fuel during combustion. Compression is important because compressing the fuel and air mixture before combusting it is the only way for it to burn fast enough to make any real usable power. Last but not least we need an ignition source to light the whole works off.

Air/Fuel mixture

In a gasoline engine it is very important that for every bit of fuel in the engine there is 14.7 bits of air mixed with it. This ratio is called a “stoichiometric” ratio, which means that it’s the perfect amount of oxygen and fuel so that after it’s all burned up there’s no more fuel and no more Oxygen. Well, there’s oxygen but it’s not in the form of O2 which we breathe… but that’s for the chemistry section. There are, however, times when variation of that ratio is important for combustion stability and for efficiency. For different kinds of fuel the stoichiometric ratio is different, like methane which has a stoichiometric ratio of 17.2:1 and Diesel engines which almost always operate in “lean” conditions where there is extra oxygen left over after all of the fuel is burned.

Compression

Compression is vital for internal combustion engine operation. Modern gasoline engines compress the air/fuel mixture to somewhere between eight and eleven times the ambient pressure outside of the engine. The fuel and oxygen atoms in this compressed mixture are closer together and the temperature is much higher than before it was compressed. With these conditions ignition of the fuel/air mixture can happen faster than in normal pressures which allows more of the energy of the explosion to be used than if the ignition happened at lower pressures and temperatures. Without compression the ignition would happen slow enough that the ignited fuel wouldn’t be able to light the surrounding fuel fast enough to keep the chemical reaction going long enough to burn all of the fuel.

Ignition Source

 There are two ways to ignite the compressed fuel/air mixture inside the engine once it’s ready to burn. One is by introducing an electric spark to ignite the fuel. The other is from the compression we were talking about earlier which is called auto-ignition. Auto-ignition is when there is enough heat made during compression that the fuel/air mixture spontaneously combust. Some fuels like diesel are much more predictable during auto-ignition than gasoline. Diesel engines do not have spark plugs, but rather, the fuel is injected directly into the pre-compressed air so that it lights off immediately after being vaporized during injection. This kind of ignition is more violent and louder than spark ignition, which is why diesel engines are so much noisier than gasoline and have that characteristic “clack clack clack” sound when running. Since diesel is ignited this way the amount of compression in a diesel engine is usually much higher than a gasoline engine which contributes to engine efficiency.

Throttle the Chaos

Gasoline engines need to have very good control of the amount of air coming into the engine since the amount of fuel must always match the amount of air. So in gasoline engines there is a throttle plate in the intake of the engine which is controlled by the throttle pedal which you push to make it go. Diesel engines can have extra air in combustion so there is no throttle plate on a diesel engine. The throttle pedal in a diesel car only controls how much fuel is added and the air is not controlled as precisely. This is all because of the type of fuel and the type of combustion which is happening. Nice to know huh?

Chemistry as Promised

A hydrocarbon chain is basically a chain of carbon atoms surrounded by hydrogen atoms like this picture of a diesel molecule with 16 carbon atoms and 34 hydrogen atoms. The differences from one fuel type to another are mostly determined by the length of this chain. For instance, Methane is the shortest chain of one carbon atom and four hydrogen atoms, gasoline has eight carbon atoms and eighteen hydrogen atoms. This is where the name hydrocarbon comes from, they’re all made of hydrogenated carbon chains.

The Diesel hydrocarbon molecule

So hopefully you learned something from all of this. I have about three more pages of details that I keep having to cut out just to get the main points across without writing a book. I’ve got no shortage of content, that’s for sure! more blogs to come! 🙂