Adding boost to high compression engines

In reply to STM317 :

Most of the time, one. 

During the cold start where emissions really matter, as well as generating heat 2 or more.  This is the time where you will see compression stroke injections.

And there are some instances where at high load, you will see two injections.  But I've never seen that other than intake stroke injections.  But +99% of operation will be one single intake stroke injection.

The nice thing about gas engines, assuming that you have a decent amount of emissions generated, the actual limit isn't that important- as the catalysts work exactly in the range that the engines work.  And that's one huge reason that from an engine out aspect, gas engines are far dirtier than diesel engines.  Mostly about an order of magnitude dirtier.

But since the aftertreatment is so effective, the vehicle level emissions can be a lot lower for a MUCH lower price, and less combustion compromise.  And that allows gas engines to focus more on power and fuel economy when the engine gets past the first 30 seconds of operation.

I very much understand why people think DI is like diesel.  Other than HCCI, it's not- it's just a different way to put a stoich mixture of a/f into the chamber and have it burn.

In reply to alfadriver :

Interesting. Thanks for sharing!

Knurled said:

In reply to curtis73 :

It might interest you to know that Mazda has a compression ignition gasoline engine developed and going into production cars for the 2018 or 2019MY.

 

The only problem with saying something can't be done is when someone else is already doing it

 

I thought most of the Diesel's efficiency advantage was a combination of always running lean, and the flame kernel being insulated from the water jackets by the excess, unburnable endgas air.  Less heat gets lots to the cooling system and certainly the EGTs are extremely low at low loads because it's running so incredibly lean.  I've never heard of them making more torque per displacement than spark ignition engines, though.

Many diesel engines make much more torque than their gasoline counterparts per displacement.

6.7L Powerstrokes (the latest ones) make 860 lb ft.  I would challenge anyone to find a 6.7L gas turbo engine that makes 860 lb ft.

1.9L TDI makes 240 lb ft, 1.8L turbo makes 175.  That's not to say that they couldn't be equal, but the higher BTU content of the fuel and the duration of the burn are two huge factors in why diesels tend to make more torque.  This shouldn't even be a debate.  Look at diesel torque numbers versus gas for the last 40 years.  Not even debatable.

In reply to curtis73 :

Don't forget, the diesels are low revving.  So they *have* to crank them to 11 in the torque department to get adequate HP out of them.  With a gas engine, up to a point, it's easier to just rev it higher for hp and not need a super beefy (heavy) bottom end that can withstand insane torque at low rpm.  

rslifkin said:

In reply to curtis73 :

Don't forget, the diesels are low revving.  So they *have* to crank them to 11 in the torque department to get adequate HP out of them.  With a gas engine, up to a point, it's easier to just rev it higher for hp and not need a super beefy (heavy) bottom end that can withstand insane torque at low rpm.  

And one should point out that driving cars is a power game, not a torque game.  So power matters. 

Still, in terms of usage, diesels do an amazing job putting out the power that most people really use.  It's really at the peak output where gas dominates.  

In reply to curtis73 :

Diesels tend to run higher boost pressures than gas engines though, so there's not really a direct comparison.

6.7 Powerstroke:

21psi of boost = 440hp, 925 ft-lbs

Destroked 5.4L LS engine:

20psi = 1100hp, 890 ft-lbs

Sure, it's a stock engine with a warranty and emissions complaince vs a one-off, but we can see that a gas engine with 1.3L less displacement, and 1psi less boost was nearly able to generate as much torque as the diesel, and nearly 3 times the hp. You may be able to find a more direct comparison, as this was just a quick search.

We could also compare it to a 6.2L Hellcat:

11.5psi (from a power robbing, belt-driven blower) = 707hp, 650 ft-lbs.

If one were to increase displacement equal to the Powerstroke, and boost up to 21psi I don't think it's a stretch to think that the numbers produced would eclipse the Powerstroke. Again, it has no warranty at that point, but numbers are numbers. The bottom line, is they're different tools for different jobs.

curtis73 said:

Many diesel engines make much more torque than their gasoline counterparts per displacement.

6.7L Powerstrokes (the latest ones) make 860 lb ft.  I would challenge anyone to find a 6.7L gas turbo engine that makes 860 lb ft.

1.9L TDI makes 240 lb ft, 1.8L turbo makes 175.  That's not to say that they couldn't be equal, but the higher BTU content of the fuel and the duration of the burn are two huge factors in why diesels tend to make more torque.  This shouldn't even be a debate.  Look at diesel torque numbers versus gas for the last 40 years.  Not even debatable.

 

I can easily find a turbocharged 6.7l V8 that makes more than 860ft-lb.  And it will only take 8-9psi of boost, and not 20-30psi or whatever the 6.7 does.

Compare nonturbo to nonturbo engines.

Many diesel engines make much more torque than their gasoline counterparts per displacement.

6.7L Powerstrokes (the latest ones) make 860 lb ft.  I would challenge anyone to find a 6.7L gas turbo engine that makes 860 lb ft.

1.9L TDI makes 240 lb ft, 1.8L turbo makes 175.  That's not to say that they couldn't be equal, but the higher BTU content of the fuel and the duration of the burn are two huge factors in why diesels tend to make more torque.  This shouldn't even be a debate.  Look at diesel torque numbers versus gas for the last 40 years.  Not even debatable.

One of the dirty little secrets that diesel enthusiasts don't want you to realize is that diesels completely suck ass without boost, and if you compare to gas engines with the same displacement and boost pressure, the gas engine will make similar torque and probably nearly twice the power. But, there are many caveats and extenuating circumstances. In a gas engine, fuel is always proportional to air flow because it's almost impossible to ignite an air/fuel mixture outside of a narrow range that gas engines must always stay in. On a diesel engine, you can force more air through it without adding a proportional amount of fuel and it will still run, so boost pressure on a diesel does not necessarily equate to burning more fuel and making more power. Likewise, comparing power numbers from gas to diesel is meaningless if you don't consider the rpm difference. Regardless, it is possible to make the same power and torque from a gas engine as a diesel with the same displacement and boost pressure. However as i mentioned earlier, diesels are literally immune to detonation in the gas engine sense because they do not inject ALL of their fuel before the ignition begins,  so diesels are much more forgiving and less sensitive to the vagaries of the outside world. It is not hard to imagine a 6.7L gas engine which can make 850 lb ft at 3000 rpm, but then blows up if ambient temps outside go up by 30F. 

curtis73 said:

sesto elemento said:

Interesting about the preignition, I must've been misinformed. It would seem to me that you could benefit greatly by not having fuel present until the last possible moment to prevent preignition but maybe the pressures needed to overcome the pressure differential would be difficult to achieve.  

What you are describing is a diesel engine, except you're still using a spark to ignite.  The flame front speeds of gasoline prevent this from being really feasible.  It works on diesels because of the slow burning fuel (also why diesels tend to have larger torque numbers as the flame front continues to exert force on the piston during a larger arc of the crank throw.  Gasoline engines are pretty much done pushing by 20-25 degrees ATDC, diesels more like 50-80 degrees).

Getting the injection timing correct (we're talking milliseconds) for gasoline to operate like a diesel is nearly impossible.  Even if you could get a base map established, the processing speed of the ECU couldn't keep up with adjusting it.  In the time it would take a knock sensor to tell the ECU, the ECU to process it, and the mechanicals to respond you would have already melted 2 pistons.  Diesels are pretty easy.  Throw in some fuel and it burns slowly.  Gasoline goes boom really fast. (also one of the reasons diesels are limited in their RPMs. Its hard to get all that fuel to burn in the shorter times you give it with higher RPMs)

Even with a diesel, the reason they sound like they do is because they are basically operating every combustion event as if it were a controlled ping.

What you point out about slower flame speed of diesel compared to gasoline had a ringing note to it.

Alcohol has a slower speed than gasoline and nitromethane  has a slower speed than alcohol.   I think toluene  is even slower still.  

I was trying to work out how much in crank degrees but my poor math skills failed me.  Nevertheless there is some real torque and power advantages with slower flame speed 

Then I thought I could back door it  by comparing known dyno figures  with each other and that’s where I got bogged down. 

In reply to frenchyd :

The problem is, if you get the flame speed too slow, you run into the diesel problem where it costs you power by significantly limiting how high you can rev the thing. 

rslifkin said:

In reply to frenchyd :

The problem is, if you get the flame speed too slow, you run into the diesel problem where it costs you power by significantly limiting how high you can rev the thing. 

I don’t think so,  Indy cars rev pretty high, sprint cars as well.  Those are just off the top of my head. I suppose we could also include NASCAR  since they use alcohol in their fuel as well. 

Then there is the whole Formula one thing which was using fuel high in Toluene for a while. 

I personally like a slower flame front. I grew up racing Jaguar’s with their 4.17 inch stroke and admiring  Offenhauser’s with it’s up to 4.5 inch stroke.  

In their day  both were real major winners

wspohn said:

Yeah, the DI has significant cooling effect and allows you to run more boost that yu would otherwise.  I see 25 psi+ on my Ecotec.

DI also has it's own issues - carbon deposits on intake valves, and with a turbo engine you can't run any sort of cleaner through the throttle body, or it can kill the turbo bearings.

Even non DI engines can have significant boost with modern engine comtrols - an S54 BMW has 11.5 comprssion ratio and can still be turboed 

https://youtu.be/silQi_8x3gw

 

https://www.bimmerforums.com/forum/showthread.php?2123962-E30-S54-TURBO-1060-WHP-1130Nm-(833lbs)-1167WHP-E21

I’ve given the deposits in direct injection engines some thoughts. Deposits have to come from someplace.  I’ll assume that the air injected is clean, some sort of filters,ie no chunks of dirt.  

Then the deposits have to be a result of fuel.  Maybe fuel isn’t being directed down the intake anymore, but we all know about valve overlap.   We know what cleans deposits.  Alcohol!   

A fuel rich in alcohol will clean deposits. 

Knurled said:

curtis73 said:

Many diesel engines make much more torque than their gasoline counterparts per displacement.

6.7L Powerstrokes (the latest ones) make 860 lb ft.  I would challenge anyone to find a 6.7L gas turbo engine that makes 860 lb ft.

1.9L TDI makes 240 lb ft, 1.8L turbo makes 175.  That's not to say that they couldn't be equal, but the higher BTU content of the fuel and the duration of the burn are two huge factors in why diesels tend to make more torque.  This shouldn't even be a debate.  Look at diesel torque numbers versus gas for the last 40 years.  Not even debatable.

 

I can easily find a turbocharged 6.7l V8 that makes more than 860ft-lb.  And it will only take 8-9psi of boost, and not 20-30psi or whatever the 6.7 does.

Compare nonturbo to nonturbo engines.

From the factory?

Vigo said:

And then there's the benefit of direct injection! Port injection sends fuel in on the intake stroke, which means the fuel heats up along with the air during the compression stroke. Injecting fuel directly into the cylinder when most of the intake stroke is past means the fuel didn't heat up from contact with the compressing air for as long.  Take the benefits of direct injection a little further and imagine the case with diesel engines. They are all direct injection and unlike gas engines which have all the fuel present at the time of ignition, diesel engines inject the fuel 'gradually' into an ongoing combustion, so it is literally impossible for them to experience detonation in the way gas engines do. I think this is one of the

This is the biggest factor, surprised it took this long for someone to mention it. The Toyobaru's engine wouldn't even hold up well in NA form without DI.

Edit: Whoops zombie thread. Still, remember when that kind of compression worked best when running on ethanol?

In reply to frenchyd :

I'm pretty sure those fuels don't cross the threshold of "too slow" for flame speed like diesel does.  

In reply to rslifkin :

You’re probably correct although I’m not sure about Toluene 

from the "net"

Toluene is one of the higher-value molecules already in use in many gasoline blends. The main benefit is that it's an octane booster that does not dilute the energy content of the gasoline like ethanol does. It's a C7 aromatic molecule, which means it contains a lot of energy and vaporizes easily, but not so easily that it's an air pollutant like lighter benzene (C6 aromatic) and butane (C4) molecules, which percolate out of unsealed gas tanks and contribute to smog.

The tricky thing about adding aromatics is that the total quantity in your fuel shouldn't exceed about 30%, which is the sum of all the benzene, toluene, and xylene put together. Many regular gasoline blends already contain significant amounts of aromatics -- up to 30%! -- so there's no simple way to tell how much more you can safely add. As it happens, the higher quality gasoline blends (which supposedly includes Shell and Chevron gas but don't quote me on that) already contain a lot of toluene and xylene. Adding toluene is more likely to improve low-octane gas than high-octane gas. 

A rule of thumb floating around the internet is that you can safely add 10% toluene to your gas without causing engine issues. So that would increase your 93 octane gas to (93 * .9) + (114 * .1) = 95.1 octane.

But please make sure you really need higher octane! Read this first: http://en.wikipedia.org/wiki/Oct... or one of the many questions about octane. Toluene is a better octane-booster than many but it's not going to help if you don't need higher octane to start with.

My team found on the dyno when running a low compression restricted class we made more power with lower octane fuel.

From another article about bathtub chemistry

A. Benzol - A high octane ( 108 approx. ) hot - burning petroleum product.
Although benzol will auto-ignite, it serves as a coolant when added to the 
fuel mixture ( 10% by volume).

B. Ether - A highly questionable racing fuel. Although generally used as an 
igniter, it serves best - if used - as a starter fraction.

C. Acetone - Used generally as an igniter to boost top and RPM and 
acceleration. Has strong solvent action on plastics.
Acetone is also used as a blend stabilizer, plus a water tolerance
booster ( methanol blends, with 1% to 3% of distilled water added to 
mixture). 

frenchyd said:

In reply to Trackmouse :Well said Track mouse.  The higher octane of alcohol ( 114 for ethanol 116 for methanol) plus the cooling charge of alcohol really increases the allowable cylinder pressure and thus the potential for power gains.  

Yeh!!    E85!!!!

 

The octane for E85 has been debated and it seems it's actually way lower than 114, more likely in the high 90's. That said, its ability to cool the charge is a separate (and highly desirable) feature for FI.

I really wish people would stop equating higher octane with slower flame speed.  That's not how it works.  Octane is more a rating of the stability of a molecule under high temperature and pressure than how fast a flame moves.

Especially when the flame speed of a air-fuel mixture is far more dependent on the turbulence in the combustion chamber than the laminar flame speed of a chemical burning.

You can use toluene not only as an octane booster, but as the primary basis of the fuel for a gasoline-powered car:

https://www.f1fanatic.co.uk/2007/02/15/banned-rocket-fuel/

You'll need a fuel preheater though, and you'd have to re-tune the engine since stoich AFR for toluene is 11.5:1 and peak power is 9.8:1. Which gets awfully close to the rich limit of a wideband O2 sensor intended for a gas engine.

GameboyRMH said:

You can use toluene not only as an octane booster, but as the primary basis of the fuel for a gasoline-powered car:

https://www.f1fanatic.co.uk/2007/02/15/banned-rocket-fuel/

You'll need a fuel preheater though, and you'd have to re-tune the engine since stoich AFR for toluene is 11.5:1 and peak power is 9.8:1. Which gets awfully close to the rich limit of a wideband O2 sensor intended for a gas engine.

Uh, that only applies for gasoline.  9.8:1 on tolulene is a lambda of .85- well lean of where a O2 sensor has problems.  The O2 sensor really gives you lambda, not a/f. 

In reply to GameboyRMH :

No, you won't hit the rich limit on an O2 sensor.  You'd hit the rich limit if you were running on gasoline at 9.5:1, but not on toluene.  The O2 sensor only measures residual air content in the air (lambda ratio), which is then multiplied by stoichiometric AFR to give the actual AFR.  If you switch fuels, the gauge will then read the wrong AFR, but the correct lambda.  ANY fuel running at stoich will result in a lambda of 1:1, which will display as 14.7 on a gauge designed for use with gasoline.  Some gauges let you select what the stoichiometric AFR for your fuel is, so that the gauge reads "correctly" when displaying AFR instead of lambda.

If your gauge can't read richer than 10:1 on gasoline, it could read as rich as ~6:1 on toluene because those are the same lambda value, but for different fuels.

Edit: Alfa, you beat me to it.

In reply to kb58 :

That’s true about E85 some is mixed will low grade asphalt ( insert smiley face here to indicate joke) and ethanol while most of the stuff from people like VP fuels is mixed with higher grade gasoline.  

frenchyd said:

In reply to kb58 :

That’s true about E85 some is mixed will low grade asphalt ( insert smiley face here to indicate joke) and ethanol while most of the stuff from people like VP fuels is mixed with higher grade gasoline.  

Which is why one should learn to distill their own alcohol for fuel.  Mix E100 with even 91RON E10, and you will still end up with E85 well over 100 RON.

In reply to alfadriver :

You are correct but we are trying to explain things on the internet usually with a hand held device and tend to use short cuts even if not technically correct. 

I think what they are saying is that certain chemicals reduce the tendency to detonate rather than really trying to discuss flame speed.  

I know I tend to use them somewhat interchangeably     While I really think I understand the difference.