Mike Murphy

I understand that diesel fuel has more energy content and burns much faster than gasoline. You would think this would be an ideal choice for fuel in performance engines. Aside from diesel engines generally having more mass and longer stoke than a gasoline engine, the diesel engine should be able to produce more power (not just torque) than a gasoline engine.

Audi's R10 engine had a usable RPM power range between 3000 and 5000 RPM. My thought was that the engine was too heavy, or the stroke too long, but it seems that the real reason is due to the restrictors size. If the restrictor was taken off, the engine could produce over 700hp. Interestingly, the exhaust filters for emissions on the vehicle would get clogged if it ran that high...

So why aren't we building more diesel racing engines to run at higher rpm ranges?

insite

the temperature required to compression ignite diesel fuel is only reached at a very high compression ratio. this compression ratio can only be reached by using a long stroke. the rod angles introduced with such a long stroke are not condusive to high RPM driving. since power is (torque * RPM) / 5252, power is heavily dependant upon RPM. if your RPMs are limited, as in a diesel, your power will be limited as well.

RedlineMan

I'm no engineer;

But is not gasoline far far more volitile? I imagine the reason you can't get the rpm from Diesel is that the firing/combustion timing cannot be controlled tightly enough.

the90

Flame speed in the combustion chamber increases proportionately with rpm in a SI gas engine, this is not the case with a CI diesel motor, where flame speed is roughly 'fixed'. Hence the RPM limit.

mark kibort

also keep in mind, that its all about power!
acceleration= power/(mass x velocity

What this means is that when comparing the acceleration performance of any two cars, is that it will be proportional to power. So, an R10 with 650 hp will accelerate almost exactly the same, at any speed, as a R8 with 650hp, even though it may have half the engine torque! in otherwords, HP will determine the actual force (torque) at the rear wheels at ANY vehicle speed.

one nice thing about the diesels, is that the low revs for equal HP could equate to increased reliabilty, but 5krpm diesels are pretty high reving for their designs.
mk

insite

correct. gearing equalizes the field here. another difference is that the R8 can use a more conventional gear box; dealing with power resulting from RPM's is easier than dealing with power resulting from brute force. the audi's torque posed quite a design challenge, i imagine.

doc2s

[QUOTE=mark kibort]also keep in mind, that its all about power!
acceleration= power/(mass x velocity

/QUOTE]

power=force x velocity. force is what causes the acceleration. velocity is a result of that acceleration over time. hence, force dictates what the power is and not vice versa.

mark kibort

not exactly. First of all, you are referring to the "force" or torque at the rear wheels, which IS dictated by power. power, regardless of comparitive engine torque, will dictate the rear wheel torque. (which will also be the same at ANY vehicle speed for two cars with the same Power, but different engine torque values)

so, yes, force causes the acceleration, but power determines what amount of it you have.

acceleration=power/(mass x velocity)

this addresses the common misconceptions about power.

to address your statement below, force at a given rpm, will dictate what power is, NOT just force.

per the equation, acceleration is proportional to power and will be inversely proportional to velocity. (ie as speed goes up, acceleration goes down as long as power is constant. close ratio gear boxes allow for relatively constant power application)


Power WILL dictate the force at a given speed.
force x distance is work. POWER is the rate of doing work. ala Energy!

Mk


[QUOTE=doc2s]

Mike Murphy

Agreed on all points. I'm wondering how a diesel engine would perform at say, 8000 RPM if the internal components could somehow withstand the high internal piston speeds and mass of long-stroke components.

Mike Murphy

Gasoline has about 150,100 BTUs of energy per gallon. Diesel has 166,600 BTUs of energy per gallon. Regular diesel has a much higher octane rating than regular gasoline as found at the pump.

Nordschleife

Ulrich Baretzky tells me that the engine is capable of 'much more' bhp than you envisage. It also revs higher.

We really don't want more diesels in racing, they are boring, boring, boring. I llove my diesel AWD off road, I like my Smart diesel for put-putting round town, but on the open road or the track, they are booooorrrriiinnnngggg. Imagine getting a C-GT and being forced to change gear at 4,000 rpm all the time.

Just ecause something can be done, it doesn't mean it should be done.

R+C

Mike Murphy

That may be true, but I think diesel burns about 8 times faster than gasoline, so I'm wondering if that really is the limiting factor?

lawjdc

I think you are assuming that the Audis are burning the type of diesel fuel you and I can purchase at the pump. Its not. In fact, its a clear liquid. The Audi race motors are diesel motors, but the fuel definitely is not diesel fuel.

Geoffrey

What exactly are we comparing here, the ACO rules of compression ignition engines vs spark ignition engines, or are we simply comparing the two in general terms?

The combustion process of a compression ignition engine requires the engine to be run with an excess air factor (lean condition) compared to a spark ignition engine. Where a spark ignition engine runs in the 13:1 range for a N/A engine and a 12:1 range for a forced induction engine, a compression ignition engine runs in the 18:1 range. This is largely due to the rules of "no visible particulate emissions". In a paper comparing the two types of engines, it was stated that for equivelant power, a compression ignition engine would require a restrictor that is about 20% larger than a spark ignition engine.

Additionally, the current compression ignition racing engine technology is very new and has not been exploited or pushed to the edge compared to spark ignition engines where the technology is very mature (NASCAR & F1 for example). Over time, you'll see the air fuel ratio become optimized against the particulate emissions which will help to find the real world power availibility. There are other design aspects like valve angle, combustion shape that both help the combustion process efficiency as well as reduce visible particulate emissions that have not yet been optimized, unlike spark ignition engine technology. Newer electronics such as variable flow rate injectors will help optimize fueling since the injection window is much smaller than in a spark ignition engine.

As mentioned above, the Shell racing fuel has a Cetane number of 62 because it is a formulated fuel with a high (almost 1/3) liquid to gas synthetic which helps reduce the visible smoke. Normal diesel has a Cetane number of 50.

doc2s

[QUOTE=mark kibort]


Power WILL dictate the force at a given speed.
force x distance is work. POWER is the rate of doing work. ala Energy!

Mk



power is indeed the rate of work done BY THE FORCE/Torque. hence the causality is that force/torque dictates the power and not vice versa.

your equation is just plugging in the definition of power in newton's law to eliminate the force/torque. if your velocity is rotational then it should be torque instead of force.