Downsized charged engines, peak cylinder pressures? 5

[HarleyE30]

Hi guys,

Just wondering what typical peak cylinder pressures are for these new breeds of downsized turbocharged gasoline engines we're beginning to see ?

Cheers,

Harley

 

[decipha]

yes every engine I've ever turbocharged MUST have a wastegate otherwise they will come apart (as in rotating assembly / engine block failure), typically most engines burning 93 octane pump gas don't run anymore than 20 lbs of boost


The more efficient you make an engine at a given state, the more performance and economy you will return at that state.

This is currently one of the primary 'pushes' in the industry and why a lot more vehicles are being manufactured factory turbocharged in an effort to increase economy and as a result performance as well.

 

[Lou Scannon]

This is currently one of the primary 'pushes' in the industry and why a lot more vehicles are being manufactured factory turbocharged in an effort to increase economy and as a result performance as well.

Umm, yeah, we all understand how downsizing with turbocharging works to increase fuel economy and performance as well (due to reduced vehicle mass, pumping losses, & engine friction resulting from downsized engine). Question on the table is what is the physical explanation for increases in fuel economy you claim as follows:

In my experiences, turbocharging small block Ford's in 3800 lbs cars, fuel economy usually increases from 22-26mpg N/A to 28-31 mpg under similar cruising conditions...no other changes all else remaining constant

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

Yes, this is key. When you replace a 3.8L with a turbocharged 2L you may get greater economy. This is because at zero boost the pumping losses through the smaller engine are less. Also, engine friction is less, the engine package and car are lighter, etc. But, when you take a 3.8L and turbocharge it, you usually get lower economy, especially if the engine is "overboosted" meaning it requires a waste gate to bleed off exhaust back pressure to keep boost under control.
So, if you are getting the kind of gain in economy you say you are getting, your installation would be of extreme interest. Can you please describe in detail?

 

[decipha]

I misinterpreted your question

the gains are accredited to the reduction in pumping losses

In theory you have pumping gains from the exhaust heat energy used to spool the turbine which would otherwise be pumped out of the exhaust pipe with the majority being wasted energy

 

[140Airpower]

Well... to recover efficiency from the exhaust at cruise is pretty hard to do. Long haul diesel trucks can do it because they don't run engine vacuum. The compressor can make pressure that pushes down on the piston, but the back pressure in the exhaust side has to be significantly lower or you don't get a net advantage. So, these are low back pressure installations.
Auto installations are high back pressure so you can have quick turbo response. They have waste gates to limit boost on top. It's a different approach. Even with diesel auto engines I'd like to see how it's done for increasing cruise efficiency. I bet variable nozzle geometry is where you start.

 

[decipha]

no automotive engines are not high back pressure. They are designed for optimum delta pressure, the exhaust is designed to scavenge using the venturi effect, this is how horsepower gains of up to 10% can be ascertained by optimizing the exhaust manifolds which can be seen in the tri-y header design

The same applies for cylinder filling using airflow modeling and valve timinge this is why properly designed naturally aspirated engines are capable of exceeding 110% volumetric efficiency.

While the turbocharger isn't creating positive pressure (boost) during cruising conditions it is still supplying high velocity air. This is evident by removing the compressor discharge piping while the engine is running, you can feel the airflow by placing your hand in the airstream, the turbocharger (if its properly sized) will be moving sufficient airflow to perhaps dry your hands. Some higher horsepower engines have larger turbocharg that do not even spin at idle... how's that for back pressure?

 

[140Airpower]

"...no automotive engines are not high back pressure..."

This is relative. There are low back pressure systems that work primarily on exhaust gas velocity. These are called blow-down systems. The alternative is the pressure system. I have never seen an auto installation that was not a pressure system. One guarantee that a system has appreciable back pressure is the presence of a waste gate. Waste gates work by bleeding off back pressure and they work best, most precisely and over the greatest range when the back pressure in their absence would be very high. This implies that the turbo itself is a severe exhaust restriction and the combination of turbo, waste gate and associated plumbing is more restrictive than a good NA exhaust system.

"While the turbocharger isn't creating positive pressure (boost) during cruising conditions it is still supplying high velocity air".

This is what is inconsistent. A gas engine at cruise is highly throttled, producing a small fraction of its WOT power. It runs a manifold VACUUM. A turbo producing "high velocity air" is fighting the throttle while costing power in the form of back pressure.
It is very easy to gain power from adding a turbo to an existing NA engine, but gaining mpg? and especially in the amounts you claim? You have not provided details of how this was done.
I believe you would need to do mods to the NA engine, probably including exhaust gas recirculation and direct injection.

 

[decipha]

My android just refreshed the page and caused me to loose a well detailed and thorough reply so ill summarize

Exhaust delta presure is vital to proper engine operation, and turbos are restrictions but the wastegates primary purpose is not to limit back pressure although thats the effect, the wastegates purpose is to bypass exhaust gas from overspooling the turbo to limit compressor airflow, often the wastegate vents to atmosphere

The wastegate only opens when the reference line (typically manifold pressure) exceeds the diaphragm spring pressure (desired boost pressure) this usually occurs at approximately 4000 rpm at WOT for the majority of the engines I play with. Some engines where boost is increased to the compressor maps surge limit, the wastegate never opens. Typically a properly sized turbo should flow at least 300 cfm more than what the engine will be ingesting.

As for throttle angle, a typical 4300 lb vehicle (loaded with typical payload) maintaining a 70mph cruise with a slight headwind of 1-3 MPH and a vehicle drag coefficient of 0.31, final drive ratio of 2.07:1 with approx. 22% drivetrain loss at an engine speed of 2000 rpm, the engine ingests a steady 52 cfm at 20% throttle, this was at 18 ft above sealevel engine coolant temperature of 210 degrees F and aircharge temps of 122* F barometric pressure approx 30.08, humidity of 87% with ambient temperature of 83* F, this was averaged over 24 miles (causeway bridge) on 3 different days of similar weather.

 

[decipha]

engine vacuum of 14 in/hg during these conditions

 

[decipha]

oops I left a lot out, that engine in question is a 231 cubic inch 6 cylinder that averaged 19mpg prior (naturally aspirated) to the turbo and 24mpg after, again that was averaged over the 24 mile bridge.

No other changes were made

That same engine with a supercharger averaged 22mpg

 

[Tekton]

If the vehicle in question is tested under near identical part load conditions, the N.A. and Turboed cylinder pressures will likewise be near identical

Since operating the turbo creates a net loss of energy through heat dissipation and the exhaust pressure required to rotate the turbo, yet produces the same cylinder pressure as the N.A. version, how can that loss of energy be improving rather than reducing the vehicle's economy?

I concede such may be possible if the turboed engine's componentry and tune was optimised but, apart from removing the turbo, was left unchanged for the N.A. test - -

Maybe you've omitted key information, such as the N.A. engine was a larger capacity unit than the turboed engine - -

Tekton

 

[decipha]

I don't believe I can be of any further assistance, I don't have a definitive answer for you, all i know is the real world results

 

[Lou Scannon]

We'll, if you're going to play that card, then can you share your methodology for determining that your observed delta fuel economy measurement is statistically significant?

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

decipha,
You have to have done some special and unusual things to get the results you claim. Otherwise, what you say is not credible.

 

[Marquis]

Typical PCPs of GDI boosted engines I've seen are like early diesel motors- 110 -130 bar.

Regarding turbocharging increasing fuel economy with no mods-

when did the workshop-mechanics equipped with dremel and seat of the pants dynos invade these forums?

http://www.auto-scape.com

Sideways To Victory!

 

[decipha]

I gave up on the debate because there's nothing more that can be said, the fact is engines get better fuel economy when they are fitted with turbochargers as well as superchargers, this cannot be disputed its a known fact by millions of racers around the globe, if you disagree you are wrong its a proven fact not for debate

its statistically significant because the vehicle can travel a greater distance without refueling, one of the primary factors of engineering design is efficiency which comes only second to safety

As for not being credible, I'm appalled you would even insinuate I'm being dishonest, what benefit could possibly be ascertained from spreading mistruths?

Perhaps you aren't aware that the knowledge we gain in life is only temporary, when we pass on that knowledge does us no good.

I've tuned several hundred engines I have yet to have a single 1 get less fuel economy than it did prior to being boosted, in other words I have yet to have a single engine not get better fuel economy when boosting abilities were added.

As I said prior there's nothing more that can be said, if anyone cares to elaborate as to the physics behind the increase in economy then go right ahead otherwise your spreading mistruth about something you cannot comprehend.

 

[GregLocock]

My quiet bet is that you did a bit more than just 'fit' a turbocharger. I suspect you played with the mixture and the timing at least, Normally in order to get a 25% boost in fuel economy you'd have to change the gearing.

At 20% throttle your turbo will not be doing anything much.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[140Airpower]

Turbos and superchargers are not add-on economy boosters. They are power boosters. Normally you will lose economy from such installations. The simplest example is adding a supercharger with no mods to the engine.
First, the supercharger will increase the total compression of the engine, fully equivalent to increasing the CR. It does this because it increases the air intake per cycle while the combustion chamber volume remains the same. So, it can result in the necessity to use a fuel with a greater octane number. This means that fuel costs may increase and the engine's ability to take supercharging is ultimately limited from this effect.

Assume we take a 3.8L engine and add a positive displacement supercharger (like a Roots type) that increases the breathing capacity by 20%. Now the engine has a volumetric capacity that is 20% greater than before. For all intents and purposes it behaves as a 4.56L engine. That means that at cruise it has the pumping losses of a 4.56L and a pure and unavoidable decrease in economy compared to its unsupercharged state as a 3.8L.

However, there are things that can be done such as what Greg mentions, lowering the gearing. Also, you can declutch the supercharger and bypass it so that the engine behaves again like a 3.8L, but that has a lower gearing. NOW you can gain economy. But, it takes details like that that can make something like this work. The supercharger alone "with no other changes" LOWERS economy.

 

[decipha]

that is incorrect, boosting an engine does not require a higher octane fuel, the higher octane fuel is often used to increase power by the knock suppression it is not a requirement of any internal combustion engine

from your logic you are failing to comprehend the dynamics of a running engine, fuel consumption is not linear to engine size, there are thousands of 350+ cubic inch engines that better 31mpg in a 3800lb vehicle, yet there are thousands more 4 cylinder and 6 cylinder engines less than 200 cubic inches that cannot match that fuel economy in even lighter vehicles

 

[BrianPetersen]

"Boosting an engine does not require a higher octane fuel" - Say what? Higher cylinder pressure with higher temperature (particularly if there is no intercooling) means more knock propensity.