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Diesel versus Gas

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Diesel versus Gas

94twindodge 94twindodge
New User | Posts: 5 | Joined: 12/08
Posted: 12/18/08
03:13 PM

Maybe its just me but if were to compare diesel engines and gas engines I would compare diesels with big blocks or v-10s

I always understood that a diesel got small block fuel economy with big block power

just my $.02  
94 Dodge Ram 2wd auto 246k bosch 370s schied high flow delivery valves Haisley machine injection lines billet converter thick flexplate 5in straight pipe to stack pdr s300 twins afc spring kit kdp fix custom fuel plate and 2 bald back tires

Edward A. Sanchez Edward A. Sanchez
Administrator | Posts: 2112 | Joined: 07/06
Posted: 12/18/08
03:45 PM

Roughly speaking. Unloaded a small-block would be somewhat close to a diesel in fuel economy. But put a load in the back, or hitch up a trailer, and it's all over. Diesels get FAR better fuel economy under load than gas engines.  

assassin3 assassin3
New User | Posts: 25 | Joined: 04/08
Posted: 05/04/10
06:30 PM

Diesels are down on power when compared to petrol (gas) engines, but they have massive torque at lower engine revs, this is due to improved and more efficient combustion.
Diesel burns slower than petrol and it is this which gives us the additional torque, the combustion takes place much longer down the engines stroke,it is this efficiency which gives much better fuel consumption. Here in the UK (and Europe) the switch to diesel now means that more diesels are sold, they offer their massive torque in the normal driving range so do not need the high engine revs that petrols do to obtain power.

As an example, BMW diesel gets 59.6MPG and 181BHP from its 2 litre engine, the petrol (gas) gets 39MPG and 168BHP, the diesel gets nearly twice as much torque in its normal driving range, it tows more, and a much better unit.  

duskylim duskylim
New User | Posts: 1 | Joined: 04/10
Posted: 05/21/10
11:35 PM

The fundamental reasons why diesel engine do NOT produce as much power as equivalent gasoline engines are:

Diesel engines cannot burn all the air they inhale. This is due to the need to inject the fuel as a liquid and have that liquid vaporize then burn.

Note: liquid fuels do not burn - only vapors mixed with air do, thus diesel engines have the dilemma of having to vaporize their fuel in a very short period of time.

Gasoline engines have the air-fuel charge pre-mixed and the fuel is atomized by the time the spark begins combustion, an advantage not shared by diesels.

All diesel injection systems spray the fuel into fine droplets, the newest systems are able to atomize their spray very well - using very high injection pressures > 29,000 psig and tiny, multi-hole nozzles.

Producing those injection pressures requires a lot of power which saps some of the diesel engine's output.

One advantage diesels do have is easy ignition - the mixture is always rich enough to ignite in the vicinity of the injection nozzle, allowing for air/fuel ratios leaner than 125:1 on idle.

In contrast a gasoline engine must create an air/fuel mixture that the sparkplug can actually ignite - confining it to a air/fuel ratio of between 12.5:1 and 16:1.

It is the need to maintain these ignitable air/fuel ratios that requires throttling of the intake air on gasoline engines.

Now because the diesel fuel is injected as a liquid it takes time to vaporize and then burn. Here is how it happens in a classic diesel engine.

At the start of injection the diesel fuel droplets coming from the injection nozzle move very rapidly into the red-hot airstream, looking like miniature comets - the shrinking droplet head streams a vapor trail behind it as it moves through the combustion chamber.

Evaporation cools the core of the diesel fuel droplet and keeps it liquid as it loses momentum by air resistance and friction and slows to a stop, surrounded by a cloud of its own vapor.

It is these liquid fuel cores that form the soot that diesels are notorious for.

The vapor trail behind it begins to burn, starting tail-first, the tail section having mixed best with the hot combustion air.

Unlike in a spark ignition engine where combustion is initiated at a single point (the spark plug) and spreads throughout the chamber, diesel engine ignition occurs at many points throughout the fuel spray almost simultaneously.

The time period between the start of injection and the start of significant ignition/combustion pressure is referred to as the diesel's engines' ignition 'delay period'.

Slow at first, the diesel flame front forms rapidly and the combustion rate accelerates until the fuel burns as fast as the injection system can supply it.

Combustion pressure builds up so rapidly that in the final phase of injection the fuel burns almost instantaneously as it leaves the injection nozzle - producing the distinctive diesel 'knock'.

Overall, diesel combustion is actually faster than gasoline combustion. This is because the charge is denser (due to higher compression ratios) and the combustion chamber is correspondingly smaller.

The key to matching gasoline engine performance in a diesel is to mix enough air and fuel together in the tantalizingly short injection period so as to effect complete combustion.

Do it wrong and the engine just produces a lot of black smoke and soot - you have hit the 'smoke limit' of the engine.

Do it right and - that's the end of the gas engine in automobiles.

Good luck.

For the moment, we're using high-tech injection and electronic controls to make diesels mimic gasoline engine performance, but in the opinion of one of the worlds, best engine designers - Ricardo of the UK, the potential of the diesel engine has only begun to be explored.

Best Regards,

Dusky Lim
(tuner and diesel enthusiast)