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]

which means you have to reduce timing to prevent knock, you do not have to run high octane fuel you simply do not get the gains you would otherwise by not doing so, but its not required

 

[140Airpower]

Brian, you are correct. Combustion engineering is a science, not an opinion and especially not an uninformed opinion. There are text books that cover this subject in great detail.

 

[decipha]

and all those text books are wrong because you can simply reduce ignition timing to prevent knock

for example, lets say we have a 9:1 static compression ratio engine boosted making 18lbs of boost (which will return twice the horsepower the engine makes naturally aspirated), lets say this hypothetical engine reaches MBT at 36 degrees of timing at maximum pumping capacity, at 18lbs of boost we can only safely run 25 degrees of timing with the same 93 octane gasoline, the engine should return 320hp and 640hp respectively.

Now lets say we run that same engine on 87 octane gasoline, at maximum pumping capacity we can only push 28 degrees, anymore and the engine starts detonating, at 18lbs of boost we can only safely run 17 degrees of timing, the engine should return 280hp and 560hp respectively.

the octane rating of the fuel is its resistance to burning, by increasing the resistance you can run more ignition timing without detonation. A faster burning fuel can not run as much ignition timing.

disagree and you are wrong just like your text book

 

[Compositepro]

Diesel engines do not have spark plugs. When you increase compression above a certain point, a spark ignition engine engine become a compression-ignition engine. Pre-ignition and detonation are related but not the same. Both can be described as knock.

 

[BrianPetersen]

Applying turbocharging and then retarding the ignition timing will reduce power output and efficiency below what would be seen if fuel of appropriate octane rating (generally higher than the non-turbo engine wants) were to be used. Running the engine at high load with retarded ignition timing will raise exhaust temperature and exhaust valve temperature (that's where the power loss and reduced efficiency is going - it's getting thrown down the exhaust pipe, and to some extent into the oil and cooling systems, as heat). Do that extensively enough and long enough, and the hot exhaust valve will become the (pre)ignition source. When that happens ... ka-BOOM.

Just because you can control detonation (up to a point) by retarding ignition timing to later than MBT doesn't mean it solves all of your problems. The right solution is to run ignition timing close to MBT with minimum retard for detonation control, and use the octane rating that the engine wants!

By the way, I own and occasionally race a street-legal vehicle with a port-injected spark-ignition engine, non-turbo, that has high-13's compression ratio (it's not stock). I'm uncomfortable using any less than 94 octane, and when it goes to the drag strip, racing fuel. Retarding the ignition timing to try to make it run on regular is NOT happening. I prefer having the exhaust valves stay together and the coolant and oil temperatures stay in an acceptable range.

Some modern direct-injection engines, notably Mazda SkyActiv, are handling this by careful control of the spray pattern and careful design of the combustion chamber so that detonation either does not occur or remains manageable. Mine is not that sophisticated. Those engines are something of a halfway-house between gasoline and diesel engines - and Mazda is not using turbocharging in those (yet).

 

[turbocohen]

Compositepro, Pre-Ignition cannot be described as knock. Preignition is early ignition by any other means except a timed spark that normally initiates combustion. Preignition can lead to detonation and if you are that lucky then yes there is some audible knock but real preignition is silent most of the time and often leads to thermally induced damage.

 

[decipha]

pre-ignition is more of a design flaw and not so much a tuning fault

the amount of heat required to split an exhaust valve is significant, it will take a significant amount of retard to do so not to mention the engine will have to maintain that load for an extended period of time, the only real concern you have for damaged exhaust valves are in truck applications that see heavy loads during towing for extended periods of time, otherwise its null and not even worth mentioning

brian your 13:1 SCR engine shouldn't be ran on anything less than high octane fuel, but its not typical for a gasoline engine with that much compression to be turbocharged, in fact, the upper limit for boosting that i recommend is 10:1 unless you have direct injection which allows you to go higher safely

in any case, you just reiterated what i stated previously, you CAN run your engine with a lower octane fuel but it will require you to pull timing, it IS NOT required that you run higher octane fuel but you do so for safety, regardless of the fuel octane you run, you can typically reach MBT below approx 25% VE then thats when you wreak the benefits of the higher octane

i don't recommend you run your engine with low octane fuel, im just stating that you can

i've ran 450+ rwhp supercharged engines on 87 octane fuel with over 16lbs of boost

 

[Lou Scannon]

its statistically significant because the vehicle can travel a greater distance without refueling

Assess Statistical Significance

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[pickler]

for a factory honda k23 which was fitted on the acura rdx (2.3L Turbo) the max cylinder pressure was about 1400psi at 2500rpm (peak boost). subtract ~300psi when the boost started tapering off at 6000rpm. It was just a conventional 2.3L I4 with MPFI and 8.8 compression. Not sure about the newer ones with direct injection and everything.

 

[LateApexEE]

I have found that most OEM on board fuel economy gauges work off of injector pulsewidth. When fitting larger injectors the fuel economy numbers will show higher (better MPG) numbers due to the reduced injector pulsewidth at cruise. Perhaps this is where these fuel economy numbers are coming from?

 

[140Airpower]

LateApexEE, this is very interesting speculation and might help to explain the apparent violation of the laws of physics that has been claimed.

However, using the on-board fuel economy gauge to calibrate fuel use instead of a professional rig or at least calibrating the gauge or the pulse width vs amount injected and then thinking you have certifiable data to support extraordinary claims would be naive. Besides, the firm claim is that there were NO changes, so NO larger injectors.

On the other hand, applying substantial supercharging without changing injectors in several hundred cases would also seem incompetent. The risk is leaning out the engine under high power leading to catastrophic damage -guaranteed to happen in at least some of the hundreds.

 

[Tekton]

Decipha, were the economy figures you quote calculated by filling the gas tank with known volumes of fuel, which were then divided into known distance covered - - or from dash guage readouts?

Tekton

 

[tbuelna]

All modern EFI systems are closed-loop, which means that the fuel injector PWs are constantly adjusted to deliver a specific mass flow of fuel based on feedback from the oxygen sensor, the knock sensor, the air mass flow sensor, etc. As long as the PW commands are within the limits of the ECU software maps, then there should be no problem for the ECU to adjust to the higher flow rate of a larger injector.

 

[HarleyE30]

Going back to my original post, I have now finished Uni and am working in powertrain at a high-end OEM.
Peak cylinder pressures are in the region of 120 Bar.

Reading Race Engine Technology this month, Mahle have said peak cylinder pressures for the new F1 engines next season are seeing peak cylinder pressures of 200 bar, and are running in knock conditions.

 

[140Airpower]

tbuelna, that is precisely the point!!! The ECU will deliver the correct amount of fuel using a diminished duty cycle for the injectors. A meter that reports the duty cycle as a measure of fuel use will falsely report better fuel economy.

The emphatic claim by decipha of better economy in ALL installations would be more credible if he gave full details of what he did. To be thorough in making such a claim (that overturns the textbooks and the science to date on vehicle fuel economy) decipha had to have done calibration runs on roads, before and after, on each vehicle. These runs, if properly done, would not have relied on the unverified reporting of the vehicles' internal gauges. Such a procedure would have been extraordinarily expensive for the installers and burdensome in time on the customers. Short of that, dyno runs would give an inferior estimate and had to have external fuel use measuring or a recalibration of the internal mpg meter. Again, this is not an inexpensive procedure tacked onto the cost of the installation.

For these reasons, LateApexEE's observation is tantalizingly suggestive of a VERY easy mistake. On the other hand, decipha is clear that he did not change injectors "no other changes".

 

[140Airpower]

HarleyE30, how did you measure the peak pressure?

 

[HarleyE30]

@140: in-cylinder pressure measurement

 

[140Airpower]

Thanks Harley, I meant what transducer did you use? There are spark plugs with this capability and it's possible to modify a plug putting a transducer in a drilled hole. Also possible to put one in a drilled hole in the head.

 

[HarleyE30]

Im in design so am not sure of the exact method of measurement. I remember using the spark plug type transducers at uni; they were supplied by AVL. I will find out what we use on Monday!