diesel and petrol engine equivalent sizes

[RobWard]

Firstly let me explain that this question is from someone who enjoys reading this forum and is about as far removed form "expert" status as you are likely to get, so apologies if I shouldn't post it here and feel free to red flag....

I recently noticed that for a given model of car the diesel engine gets comparatively larger to its petrol equivalent (approx horsepower), as the size of engine decreases. i.e A 3 liter diesel and petrol are roughly similar, the 2 liter petrol is often equivalent to a 2.2 liter diesel and a 1 liter petrol is often equivalent to a 1.4 liter diesel.

So why is this? Do diesels become more efficient as the displacement increases, if so why, and why do petrol engines not exhibit the same characteristics.

All I could think of was that a larger petrol engine might have issues regarding the ignition source being one single spark, thus flame front propogation being a limiting factor, whereas a diesel (and I'm just supposing here) can have multiple sources of ignition as the fuel injects.

Or am I very wrong. (Just realised that weight might be a factor here)


PS I tried searching the site for the answer but couldn't find a thread on this.

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[franzh]

Where do we start:
Diesel engines are unthrottled, there are relatively few suction throttling losses;
Diesel fuel has a higher BTU content compared to its gasoline equiv;
Diesel engines typically operate at a narrow RPM range, with the exception of high-speed diesel engines for the general motoring public;
Almost all diesel engines are turbo or supercharged, mostly with intercoolers;
This is not even getting into combustion physics. All said, the diesel engine is the most efficient 'high volume' IC engine on the road. Note the emphasis on the words "high volume".

Franz

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[icrman]

Some on the reasoning is a spark ignition gasoline engine runs with a pretty much constant Air to Fuel ratio.
Where as a Diesel engine runs with an extreamly varying Air to Fuel ratio. And like someone mentioned since most diesels are not induction throttled, the way that they are throttled is by varying the A/F ratio.

 

[CESSNA1]

ROBWARD:

First be careful where you get the comparison. Automobiel amnufacturer's play all sorts of games with torque and horsepower and efficieny.

In general, and there are other reasons, a diesel gets more power per cubic inch, that is, it is more efficient is because of the higher compression ratio. Gasoline engines run around 8:1 diesel engines run around 20:1. Four cycle diesel engines can be nbaturally aspirated or equiped wtih super/turbochargers and intercoolers. Two cycle engines are generally more efficient by type (diesel/petrol) because the two cycle engines have lower pumping losses (as well as other reasons).

 

[NickE]

cessna1- Uhh I would hazard that most gasoline engines w/o turbochargers run around 10:1 compression.

 

[bobqzzi04]

It is my observation that gasoline engines have more power per cubic inch. In fact, quite a bit more.

 

[RobWard]

Thanks for the replies.
Franzh, you say that the diesel is more efficient, but why then do small engined cars have larger diesel engines to their petrol equivalents? (Renault: 1.5 diesel [65HP] and 1.2 petrol [60 HP], Ford 1.4 diesel [68HP] and 1.2 petrol [75HP]).


Rob


"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[patprimmer]

Maybe it's because the petrol engined versions of the little cars are already underpowered, so they don't want to give away as much performance by going to diesel, whereas the bigger cars have power to burn and can afford to loose a little.

Maybe the weight increase of the car increases by a greater %age in changing to diesel in the case of little cars

Regards
pat pprimmer@acay.com.au
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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[RobWard]

Underpowered?
With sixty mighty, rampant horses shoe-horned under the bonnet?

Seriously though, I see your point, but it doesn't explain WHY the diesel is putting out less power per unit displacement.

And while we are about it why do you never see really big petrol engines. When you get up to the 10 liter mark and above you only ever hear about diesels.(Well, I do anyway)

I guess what I'm asking is: what makes petrol engines more suitable for smaller aplications (small cars, motorbikes etc)and diesels better for large applications(lorries, buses, ships etc). Is it simply that in the larger applications the additional weight of the diesel block becomes a smaller percentage of the overall weight of the vehicle, or is there a fundamental limitation to the spark ignition or compression engines sizes?


"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[NickE]

I thought that the reason that gasoline engines were preffered to diesels was more to do with the power bandwidth.

 

[Aquaduct]

Some quick basics.

Gasoline combustion is knock limited. Diesel is not, so compression ratios in diesels can be 2-3 times higher than gasoline engines. Higher compression ratios increase cylinder pressures and fuel conversion efficiencies, hence the attraction of diesel. Higher cylinder pressures require stouter rotating componentry. Higher inertial masses lower engine speeds, so diesels tend to run slower. In addition, the premixed charge of a gasoline engine burns faster, so at higher speeds, combustion delays are more of a problem in diesels.

Torque is a measure of the amount of work that can be done. It is a function of mean cylinder pressure and displaced volume. Power is a measure of how fast work can be done. It is a function of mean cylinder pressure, piston crown area, and average piston speed. Getting torque out of a diesel is simply a function of making it bigger. Because of slower engine speeds, diesels have a harder time with power density (power per unit displaced volume).

In short, that's why you see 12-15L truck engines getting only 300-450 hp, the same as a car engine that might be a half or a third of the size. But the big truck engine will get about 1500 lb/ft of torque, real useful when you're trying to get 80,000+ lbs. moving. The big truck engine will also redline at about 2000 rpm. The car might be 3-4 times that.

A consequence of this is maximum speed. Because of low rpms and relatively low power, it's tough to get a Class 8 truck up much past 75-80 mph. An F1 car, on the other hand, can do 200+ mph 3L of displacement by running at close to 20,000 rpm.