Gasoline versus diesel questions

[Stelth]

My general impression of gas vs. diesel is that gasoline engines tend to have higher torque peaks, and diesels tend to have lower torque peaks. Also, that diesels are generally more fuel-efficient. It also seems that diesels have to be more heavily built, due to the compression ratios, and the low-rpm torque.

Is this correct? I know that there have been some high-powered gasoline engines that made real power at fairly low rpm's, such as the mighty Ford GAA, an 1100 cid DOHC v-8 that made 500 hp at 2800 rpm and 1000 ft. lb of torque from 1000 rpm to at least 2800 rpm. However, this doesn't get into the class of say, a diesel for a semi-truck that makes 1400 ft. lbs. of torque at 1200 rpm, and max hp at 1800 rpm. Or a locomotive engine that tops out around 900 rpm.

Are there low-speed gasoline engines that do what low-speed diesels do?

Likewise, are there high-speed diesels that act similarly to gasoline engines?

 

[simple_gifts]

OT: Mazda's Skyactive G (gas) has a 14:1 compression ratio as does the Skyactive D (diesel). The "D" is being touted as the lowest CR on any diesel built; the "G" is about as high as I've seen. Perhaps the difference really isn't that profound given the application.

 

[eljefino]

Originally Posted By: Stelth

Are there low-speed gasoline engines that do what low-speed diesels do?

Ford 300-6
Quote:

Likewise, are there high-speed diesels that act similarly to gasoline engines?


My OM613 in my mercedes 240d seemed to need revving to 3-4k to get anywhere. It struck me as a marine engine that needed to be run flat out all the time. (Marine engines are cammed to keep making more and more torque until the valves float.)

 

[sciphi]

Turbocharging improvements on small-displacement automotive gasoline engines have led to gasoline engines with low torque peaks for gasoline standards. A modern turbo gas engine can have peak torque coming on at 1500-1800 RPM and staying until high in the powerband, such as 5000-5500 RPM.

Those small, power-dense turbo gas engines are also built better than has been typical for NA gas engines.

So, the line is blurred these days, especially in passenger cars.

 

[Stelth]

Originally Posted By: simple_gifts
OT: Mazda's Skyactive G (gas) has a 14:1 compression ratio as does the Skyactive D (diesel). The "D" is being touted as the lowest CR on any diesel built; the "G" is about as high as I've seen. Perhaps the difference really isn't that profound given the application.


Not sure I'd call that off-topic. Very interesting, given the usual differences between compression ratios on gas and diesel engines.

 

[MrHorspwer]

The Bentley 6.75L twin turbo V8 in the Mulsanne makes 752 lb. ft. at 1,750 RPM. Maximum engine speed is only 4,500 RPM.

It's not a high-tech engine either. The Bentley-Rolls L-series V8 goes back over 50 years... two valves per cylinder, single camshaft, and pushrods.

 

[Chris142]

the 345 and 392 ihc run much like a Diesel. 3600 max recommended rpm and they will pull a house of it's foundation@ idle. but mpg is horrable with these engines

 

[artificialist]

It was my understanding that diesels would always be hampered by the speed at which the combustion flame travels, and gasoline engine don't have that problem as much.

 

[Stelth]

I have to admit that I get a kick out of this video:



Sounds like a delivery truck backing up.

 

[2cool]

Diesel fuel has about 20% more BTU than gasoline. In addition, the higher compression ratio (CR) would give diesel a higher volumetric efficiency. The actual time it takes for injection is the main limiting factor to diesel RPM.
As to your question on low-speed/high torque gas engines; there are SEVERAL THOUSANDS around. Ever seen a Harley-Davidson? The push-rod engines are a straight copy of the olde Liberty engine from WWI!!! High torque and low speed.

 

[Donald]

There are lots of people who alter the diesel engine to have a lot more HP.

At the 2012 Diesel Power Challenge, the winner a Ford F250 was 1635.4 HP and 3020 ft-lb torque. Wow. A lot more than my 1999 Cummins diesel.

But I want mine to last and last and unsure what I would do with 1635.4 HP anyway.

 

[Stelth]

I don't follow Le Mans, but I recently heard that Audi and Peugeot have been running diesel cars there. Anybody know any details?

 

[MetalSlug]

I seen some diesel owner is douch , I seen some blow the black smoke out tail pipe into another car behind it just for fun not just one, but 3 time. It happent to me once,I drop the gear and pass him, he try catch up, but cant.

 

[Shannow]

With direct injection allowing insane petrol engine compression ratios, and direct injection diesels dropping C.Rs, the lines between the two are narrowing.

When looked at on a straight P-V diagramme perspective, an otto cycle is supposed to be

and a diesel is supposed to be

But they are never really that disparate in reality, as the peak on the otto is always flatter, and the flat on the diesel is always peakier.

Otto

Diesel

Diesels are typically built more robust, as the first part of the ignition cycle is sufficient evaporation and heating of the edge of the injection spray to get ignition. When it goes off, there's a comparatively large volume of fuel that suddenly ignites, and causes quite a shock wave, sort of like a minor detonation event each cycle.

Modern diesels get away from that with a small injection on the compression stroke to provide heat, then multiple events through the power stroke.

To say that diesels are speed limited by injection, when the Europeans (road engines, and LeMans) have up to 7 individual injection events in a single cylinder on a single cycle isn't quite right.

Yep, diesel has more BTU, but uses them better.

My Nissan, the petrol versions got 7km/l on a run, and woeful around town. My diesel varient gets 10km/l pretty well everywhere bar gridlock, still beating petrol handsomely there (has an idle speed solenoid to keep the heater working under such conditions).

got pickies from the following sites, worth a read.
http://www.qrg.northwestern.edu/thermo/design-library/otto/otto.html
http://www.qrg.northwestern.edu/thermo/design-library/diesel/diesel.html
http://www.tpub.com/engine3/en3-15.htm

 

[MartinX]

Nice diagrams but they are quite old school, the compression ratio of the gasoline engine is 6:1 as I interpret the diagram and the diesel diagram seems to indicate no supercharging.

Modern diesel engines in cars are always turbocharged nowadays and the compression ratio is only a theoretical figure, without knowing the boost pressure it is impossible to make a comparison between engines. Diesel engines also don´t have a throttle that restricts flow at part load and this means that a diesel always runs with higher fill rate and compression pressure than a gasoline engine even if they have the same compression ratio.

Diesel engines are developing in the same way as gasoline engines, smaller displacement and higher boost pressures, but the boost pressures can be much higher than in gasoline engines since there is no risk of pre ignition, the fuel is not present before ignition.

Burn speed of the fuel is a factor and diesel burns slower than gasoline, that is the reason for the development of the modern diesel injection systems with very high pressures up to 2000 bar (29000 PSI), this results in very fine atomization of the fuel spray and increases burn speed this and electronic control allows for multiple injection events.

 

[Silk]

We have Mitsubishi Triton workshop Utes at work (Mitsi dealership) - our petrol 2.4 has 94Kw and 194 Nm of torque. The managers 2.5 diesel has 131Kw and 400Nm of torque. There is just no comparison, the diesel is just a whole new world of power and economy. Sure the diesel is turbo, it's a no brainer for diesels....a turbo petrol has too many compromises for practical use.

 

[Shannow]

Originally Posted By: MartinX
Nice diagrams but they are quite old school, the compression ratio of the gasoline engine is 6:1 as I interpret the diagram and the diesel diagram seems to indicate no supercharging.


They were intended to show that the shape of the pressure loop doesn't conform to the theoretical loops for the cycle...if that makes them wrong, then please show the correction.

Originally Posted By: MartinX
Modern diesel engines in cars are always turbocharged nowadays and the compression ratio is only a theoretical figure, without knowing the boost pressure it is impossible to make a comparison between engines.


Nope, compression ratio is ALWAYS Clearance Volume/(Clearance Volume + Swept Volume)...it's the mechanical concept.

In a turbocharged diesel engine, the "atmospheric" line is basically lifted to the average pressure of the intake and exhaust, indicating the pumping loop correctly.

You could also say that there's no way of knowing the "real" compression ratio as there is cam timing too, which is why the compression ratio is always the mechanical ratio.


Originally Posted By: MartinX
Diesel engines also don´t have a throttle that restricts flow at part load and this means that a diesel always runs with higher fill rate and compression pressure than a gasoline engine even if they have the same compression ratio.


I know of lots of diesels with throttle plates, to control engine governing, EGR,and swirl...W.O.T. which is what engine pumping curves (even the 6:1 ones, otherwise there would have been a big throttling loop on the bottom) are usually wide open, for max BMEP.

As to burn rate, in a diesel, the fuel is introduced into the chamber at a rate that controls combustion, it is not necessarily slower...most hydrocarbons have a similar flame speed when evaporated in hot air.

Originally Posted By: Silk
We have Mitsubishi Triton workshop Utes at work (Mitsi dealership) - our petrol 2.4 has 94Kw and 194 Nm of torque. The managers 2.5 diesel has 131Kw and 400Nm of torque. There is just no comparison, the diesel is just a whole new world of power and economy. Sure the diesel is turbo, it's a no brainer for diesels....a turbo petrol has too many compromises for practical use.


and then there's what he said.

 

[supton]

Static compression, yes; but dynamic compression comes into play at some point. Atkinson cycle holds the intake valve open "too long" to bleed off compression, in the search for efficency. Allows for what looks like way too high compression on paper--but the static compression won't exist in reality, and lower octane fuel is tolerated.

On my VW TDi, it does have an "intake flap", which is used for EGR duty. It also closes on shutdown, to help with engine shudder. I haven't heard of a throttle plate on a diesel otherwise; but I'm no mechanic.

 

[SteveSRT8]

Detonation in gasoline engines is greatly reduced with DI, which also avoids fuel being present in the cylinder any longer than necessary.

When I see the small super efficient sub 2 liter turbo gas engines in todays gas sippers I cannot tell their powerband from a diesel.

Also, what about the 2 stroke diesels?

 

[morris]

what makes a diesel more efficient is that a diesel doesnt limit the air flow. at a given rpm the diesel moves the same amount of air, no mater the load, or fuel used. remember 'generally' a diesel doesnt limit the air flow with a throttle plate. in a gas engine the throttle plate causes a lot of inefficiency.