Why Boost is Bad

[duncan]

A very detailed explanation to one of my purile questions elsewhere by Pat The Map

Boost is the manifestation of the reaction of an airflow encountering a restriction.... in essence boost is bad.

Take one step back and look at the Otto cycle internal combustion engine from first principles... we put air and fuel in; energy is released in the combustion reaction. In order to get more power out, we either need to extract more of the energy that is released (make it more efficient) or release more energy (increase the reactant flow rate).

There is therefore a first order correlation between power produced and airflow. So far so good...

Now a thought exercise. Try to exhale very quickly through a drinking straw. Your cheeks inflate because you generate positive pressure. Try the same exercise again with a 4" drainpipe. Your cheeks don't inflate because you don't generate any positive pressure. You are able to supply air more quickly than the drinking straw can carry it away; the same is not true of the 4" drain pipe. The drinking straw offers a restriction to the flow of air out of your lungs; the 4" drain pipe however can carry all the air away more quickly than you can deliver it. It flows better. Now let's consider equal airflow. Repeat the exercise only this time try to exhale completely in 30 seconds. No sooner and no later. It is effortless in the case of the 4" drain pipe. But it requires effort to get the same amount of air down the drinking straw in the same time. We'll look at this more closely in a minute.

The same thing happens in an engine with a turbo. The turbo tries to deliver air to the engine. If the engine can consume it as quickly as the turbo supplies it then no boost will be generated. If the turbo can supply the air more quickly than the engine can consume it then the pressure will rise. As an analogy with the previous thought experiment, consider two engines, one 2 litre and one 8 litre. The 8 litre engine will conume (assuming everything else is equal) 4 times as much air per revolution as the 2 litre; if the 8 litre engine is running with no boost then the 2 litre engine would need to be running 3 bar of boost (4 bar vs 1 bar absolute pressure) to achieve the same airflow (well, there are other effects, but for the sake of simplicity, this will do for now).

So we have established that by raising the pressure we can increase the airflow through any given restriction, and we can achieve the same airflow through different restrictions by choosing how much to pressurise the air in each instance. So what's the big deal ? If we can get the same airflow in each case, why all the fuss ?

Remember I said that it requires effort to exhale through the drinking straw ? It takes energy to compress a gas. In the case of the drinking straw that energy is provided by muscles... turbochargers are no different in that regard; it still takes effort to compress the air. But of course you don't get something for nothing. That energy has to come from somewhere. This is the job of the turbine wheel; to extract energy from the exhaust gas. The amount of energy that can be extracted from the exhaust gas is a function of the mass flow rate of the exhaust gas, its temperature, and the expansion ratio across the turbine.... we can't just magically increase the mass flow rate at a given expansion ratio (well, not on conventional turbos, you can with VGTs) so that's no good. You can't just increase the exhaust gas temperature so that just leaves us with the expansion ratio. We can't alter the pressure of the atmosphere so that just leaves us one option; we can increase the pressure of the exhaust gas before it enters the turbine. This has a twofold effect; the expansion ratio obviously increases but so does the actual mass flow rate (which is a bit of a consolation prize, but welcome all the same).

In essence, then, for a given turbo and engine operating at a given speed, the only way to increase the airflow is to increase the exhaust back pressure, which will allow the turbine to extract the energy needed by the compressor to increase its delivery rate and thus creating more pressure in the inlet. But of course, if there is back pressure in the exhaust, it becomes more difficult for the reaction products to leave the cylinders. And so a vicious cycle begins... the more airflow you ask for on a given engine and given speed the more boost you need to push the air through the engine, but also the more the exhaust pressure tries to resist. Eventually you get to a point where an increase in airflow cannot be achieved because the effect of the exhaust back pressure outweighs the gain in boost pressure... power will actually start going down at this point! Wouldn't it be just so much nicer if the engine would just consume more air at the same pressure, rather than having to raise the pressure ? That's why boost is bad.... given the choice between reducing the restriction and thus allowing the air to flow more naturally, or simply turning the boost up as a band aid for a poorly flowing engine, the better flowing engine will always be the better option.

I've only really looked at air flow here. Of course internal combustion engines are complex machines with many interactions... for example the greater mass of exhaust gas left in the cylinders after the exhaust stroke dilutes the fresh charge coming in, immediately reducing the volumetric efficiency, but furthermore, the higher mean charge temperatures thus created result in lower ignition timing tolerance thereby reducing the thermal efficiency; not only don't you get as much in, but you don't use what you do get in as efficiently!

So if we can't make the engine itself flow better, why does a bigger turbocharger help ? Well, that's fairly easy. If we make the exhaust side of the turbo bigger, then it is less of a restriction. Remember I said that it was the mass flow rate, temperature and expansion ratio that dictated how much energy could be extracted from the exhaust gas ? Well, if the turbine is less of a restriction then more exhaust gas can flow through for a given expansion ratio and temperature; we've increased the mass flow rate without the penalty of higher back pressure; or we can flow the same amount of exhaust gas at less back pressure. In the former case we make more power because we have greater reactant flow; in the latter case we make more power because we can use the same reactant flow more efficiently. It's a no lose situation in that regard. Of course the one downside is that you need some back pressure to achieve expansion; if the turbine is very open then it will need much greater exhaust gas to get it going, which equates to a higher boost threshold; the boost won't happen until later in the rev range but when it does, you'll know about it

[Gerry H]

Never one to skimp on an explanation is Pat.

Why use only one word when seventeen will suffice

[rolty]

I'll re-read this tomorrow morning when I'm a bit more sober :lol:

[Bladerider]

What a load of bollox.

How to make one of the simplest things to understand regarding turbochargers sound like rocket science.

That whole spiel can be redone as.........

Choose the correct turbo size for your application - both exhaust and compressor.

His "no lose situation" at the end which he glosses over with "the one downside is that you need some back pressure to achieve expansion; if the turbine is very open then it will need much greater exhaust gas to get it going, which equates to a higher boost threshold; the boost won't happen until later in the rev range but when it does, you'll know about it" is probably the number one priority on peoples list once they know the power level they want. TURBO LAG.

Bloke sounds like an intellectual knobber to me.

J.

[Bladerider]

I would also point out that there is no such thing as "back pressure" if he wants to get absolutely correct.

Resistance, restrictive force, or similar maybe, but theres nothing at the other end of the exhaust blowing back up it !!!! They would just equalise - positive pressure in >>>> direction meeting negative pressure in <<<< direction would just equalise out as a lesser force in whichever direction had the higher level to begin with.

Pressure going down the manifld like this >>>> meeting a restriction would ramp up the pressure created like this >>>> which is what he's talking about.

[BenTaylor]

[quote="Bladerider"]What a load of bollox.

How to make one of the simplest things to understand regarding turbochargers sound like rocket science.

That whole spiel can be redone as.........

Choose the correct turbo size for your application - both exhaust and compressor.

His "no lose situation" at the end which he glosses over with "the one downside is that you need some back pressure to achieve expansion]

And choose an appropriatly sized wastegate...

[Bladerider]

Indeed,

Im not saying that actually choosing the correct turbo is easy (or wastegate )

Just that the principles of lag, too big and too small sizings etc are pretty easy to grasp. Surge, VGT, efficiency islands, and many other things are far more complicated - I can only begin to wonder at his explanations for some of those !!!!!

J.

[duncan]

not easy for me to grasp - hence my stupid question/s to start with

[Stuart]

Next time you are at Snetterton Dunk, jump in with Gary and see if 1.8bar feels 'bad' :gay:

*******************************************

Do you really need to know the answer or are you just happy to plague people that are busier than you are, perched behind your little desk with your slippers and 'I love Scoobies' T shirt on?

I know you're an inquisitive chap and fair play to you for that, I just don't get whether you actually want to know or whether you just like to open these cans of worms and see the carnage you create................

Haven't you got something you should be datalogging? :lol:

Rant over

[duncan]

The questions wasnt whether boost was good or bad it was:

What is the difference between 1.5 bar on a small turbo and 1 .5 bar of a big turbo ?

I understand now