Efficiency of Diesel Generator.

Efficiency of any generator is a ratio of power-in to power-out.

Simply convert the power consumed and the power produced into the same engineering units (EU) then divide the total power produced by the total power consumed by the generator. (HP/HP, BTU-BTU, Joules-Joules, etc.)

Fuel consumption of a diesel generator is a direct way to measure efficiency and it is the largest cost factor in diesel driven generation.

Example: BTU content of fuel consumed by generator verses actual power delivered to the generator output terminals converted to BTU's.

Any engine that converts heat into work is limited by the Carnot efficiency:

TC is the ambient temperature (actually, the exhaust temperature) whereas TH is the temperature inside the engine.

http://en.wikipedia.org/wiki/Carnot%27s_theorem_%28thermodynamics%29

In simple speak, a lot of the heat energy is not converted to mechanical energy. You know this already because the engine gets hot. This energy is not providing motion to the wheels so the efficiency of the IC engine is usually low.

I remember doing experiments on IC engines as an undergrad but don't remember the details. The numbers you put up I don't think are far from the truth though.

There's combustion efficiency itself multiply it to cycle efficiency and transmission efficiency.

50% is quite over approximation.

The most efficient cycle(carnot cycle) operating at 400⁰C engine temperature and ambient say 18⁰C using eff. = 1-(T_c/T_h) in Kelvin would be at 56% and that is impossibility to approach.

As far as I could remember, typical eff. would be only about 28% conservative of the energy you input to your engine by fuel will be converted to actual work. The 72% of what you paid for is converted to heat loss, combustion inefficiency and frictional losses in transmission.

A good reply - because of when I was working on-site as commissioning engineer for system which required in case of emergency around 100KVA electric power. The motor of the genset was designed for 400KW power output. This always lead to discussions with the customers and complains that this engine was too big and would create too much consumption. But considering the given numbers (also given here) for the efficiency in the system chain it was easy to explain that this size was nearly correct. And there was some added margin required as in the system specifcation there was also mentioned that the genset in case of a power failure had to provide the full electrical load of 100KVA within 10sec. after power down (and we made this in 8sec. and this could only be managed with preheated motor oil in stand-by)

Within this period will a turbocharger in engine give rated output,should we use engines without turbocharger for standby/emergency application?.

Dear Mr.pnaban,

Certainly, with in this period, the Tutbo-Charger will reach full capacity. The reason being, the Static-Inertia of the Turbo Charger is less. If you tickle the Trbo-Charger, by your finger with normal force, it will rotate for few minutes with an initial speed of about 500 RPM, approx.

DHAYANANDHAN.S

I think there's some confusion here about power. You say 100KVA power needed. You don't give actual alternator rating. If the generator engine has output 400kW it could drive an alternator about 450KVA (400/0.8PF*say 90% alternator efficiency). In that case the customer is right, it is oversized, though I wouldn't expect the efficiency to be down vs a correctly sized unit by enough to be worth worrying about, specially if it's only for emergency use. If the alternator is something like 100KVA it would be an odd combination with a 400kW engine.

The 400kW engine power is the shaft output power. The energy in the diesel is something like 3 x that, 1200kW - see various posts re efficiency.

I don't know for sure whether having oversized kit makes it easier to achieve your 10 sec figure, but I doubt it.

I have read in papers that efficiency of diesel generator does not exceed 50% normally and it is between 30 to 50% most of the time. Is it true? Yes

Is the efficiency of diesel generator found my multiplying efficiency of diesel engine and efficiency of alternator? Yes

It appears that fuel efficiency also plays a vital role. Depends what you mean by fuel efficiency. Isn't it the same as efficiency of diesel engine?

Dar Mr. Codemaster,

May I clarify your doubt.?

Fuel Efficiency should mean the Combustion of the Fuel, and release of the Heat for conversion to Mechanical Energy. Alternatively mere word Fuel Efficiency may mean the physical quantity consumed. This is mainly referred as Specific Fuel Consumption.

This will be very good at full load and as the load decreases the fuel consumption will slightly increase. Best fuel efficiency will be near 88% to 90% full load which will be 0.28 Lb/Hp.Hr in FPS units, which can be converted to Kg./KW.Hr in Metric Units and it will be 0.169 Kg./KW.Hr. This power output includes the frictional loss and windage loss for diesel engine and alternator also.

This topic has been discussed in CR4 and I have furnished Specific Fuel Consumption for 25%, 50%, 75% of and near full load.

DHAYANANDHAN.S

Hello DHAYANANDHAN.S

OK thanks, but you seem to be saying there's more than one definition of Fuel Efficiency. Your 2^nd one is what I assumed in #7. This can be expressed in more than way, your 0.169 kg/kWh comes to about 48% shaft power out/fuel heat in, using typical fuel (lower) CV 44 MJ/kg. Am I right do you know in thinking a standard fuel CV is used when quoting specific power, to ensure fair comparison?

The OP asked 2 simple Yes/No questions which I gave simple answers to. Other posters gave rather more, OP may have appreciated it, but he didn't ask for it!

I still think the overall efficiency (% as per OP's question) is given by engine efficiency (%) x alternator efficiency (%). Engine efficiency can be measured on a test rig, but I don't think you can measure individual figures for your 1. and 2. As in posts #2 and #4, you could assume source and sink temperatures to come up with a (very theoretical) Carnot efficiency, and then back-calculate to get Fuel Combustion Efficiency, but it wouldn't be much help, as far as I can see. Not sure what you mean by 3. Alternators are usually direct coupled so drive losses very small, probably negligible. If you mean frictional power loss, windage loss etc within the engine it would be included in engine efficiency, if in the alternator it would be included in alternator efficiency.

Fuel efficiency could refer to the whole conversion of chemical internal energy to work, on the other hand, combustion efficiency--this has something to do with the successful pairing of C, H2 and some S with O₂ and as always this is a function of fuel atomization, excess air charge, perfect ignition timing and valve closing and opening and engine temperature.

Combustion is more efficient looking at the least amount of CO and HC (ppm) on a flue gas analyzer.

Although what you stated is at least partially true, it has not a lot to do with how hydrogen effects engine fuel economy, which is what codemaster and I were talking about, but please do expand on what you are pointing out. We are also talking more about diesel engines (that may or may not have spark-assisted ignition), and large ship engines of the type that burn BUNKER C. BUNKER C has to heated like roofing tar to even pump this to the engine. It is nasty, almost tank bottoms taken up as fuel, and is very high in C/H ratio. That is why hydrogen addition to this really cleans up the burn for one, and for the other point reduces transit fuel consumption by about 30%. This could add up to millions of dollars per year per container ship. About $7 MM USD by my calculation.

Yes, I suppose the exhaust gas could be analysed for CO, unburnt fuel etc, to find combustion efficiency, and give a clue about how to improve it, eg preheat the fuel to reduce viscosity.

Fuel efficiency is supposed to be related to the ability of the engine to totally combust the fuel supplied, and this is not nearly the case in many large engines, especially the ones that operate on poor fuels having low burn velocity such as Bunker C oil.

Diesel engines running on a high grade of diesel will still suffer from incomplete burn, or a limited burn velocity that does not allow the flame front to "keep up" with the piston during the power stroke. Hence there will be issues of not getting all the BTU's of the fuel to act in the engine cycle. This is a separate issue from the motor efficiency design point (i.e. carnot point), the generator efficiency, etc. etc.

It does factor into the unit overall efficiency, and may change depending on operating conditions, percent load on the engine, etc.

Agreed, but when an engine is tested a figure is determined for kg fuel/kWh shaft output. Whether the figure is higher than it might have been because some fuel has gone unburnt up the exhaust or due to other inefficiencies is irrelevant. Same situation at any speed and load.

You make a good point I hadn't thought of about fuel quaility. As well as CV (which probably doesn't vary much from 44 MJ/kg) need to stipulate viscosity and maybe other parameters to ensure fair comparisons.

Yep. It has been demonstrated over and over, hydrogen has the highest (that I am aware of) flame velocity, and is safe to introduce into engine intakes at dilution.

Means of generating that hydrogen have also been discussed here before, with respect to the use of scrap aluminum with catalyst and some waste heat (just for start-up) that produces the copious amounts needed without necessarily producing additional oxygen in the intake. It seems to improve burn efficiency enough to give a 30% fuel savings without adding anywhere near that much hydrogen to the intake stream. It is not a BTU oil/BTU hydrogen type of a swap. It is like adding a super-duper catalyst to the engine intake, except not a true catalyst as the hydrogen is also consumed, but I suppose one could actually condense water out of the exhaust, and send it back around, thus closing that loop as well.

Do you mean hydrogen on its own, or added to pep up an hydrocarbon fuel? If there is a significant % hydrogen (CV much higher than 44 MJ/kg ) it would bring the kg/kWh figure right down so misleading to use as comparison with diesel. Another reason why I prefer % efficiency = shaft power out/heat power in, instead of kg/kWh.

Think of it similar to a diesel fuel additive (except the hydrogen goes in with intake air, or after the turbo), as a contrubution to overall BTU consumption, the hydrogen is a small percentage, but this makes a big 30% difference in the before and after. In other words, after, one would be running on about 70% of the fuel consumed without hydrogen injection. This results in an much improved heat rate as (power in KWh/shaft work KWh), or if as you stated, you want efficiency take the reciprocal.

That is because hydrogen has higher heat content than carbon about 6 times at oxidation. But, latent heat for the by product water is deductible on it and would be minimal considering 6 times heat content than carbon.

NO NO NO!!!!! completely wrong! Nothing could be further from the truth than what you just put down in the previous post! What on earth do you base this mind numbing nonsense on?

There are derating factors due to altitude above MSL & ambient temperature.

Dear Mr. pnaban

You are right. Altitude above MSL and Ambient Temperature has an impact on the performance of the Diesel Engine.

DHAYANANDHAN.S

The diesel electric drive does not improve the efficiency of the engine it improves the efficiency of the vehicle. There is a difference between theoretical efficiency, optimal efficiency based upon bench tests in a lab or real world efficiency. Diesel engines have a theoretical efficiency over 40% but bench tests will give us results around 35%. Real world efficiency in a vehicle is substantially reduced by the transmission, the need to idle, operate at a range of speeds, operator influence and road conditions. Real world efficiency may be around 20% or less.http://www.popularmechanics.com/cars/alt...
It is a general rule that inefficiency results from changing from one form of energy to another (diesel to motion to a generator and then from motion to to electricity and then back from electricity to motion) But GM had a profitable business from 1930 to 2005 making diesel / electric hybrid locomotive engines (In its Electromotive division.) and only sold that business when it started making the petrol / electric volt based upon the same series hybrid configuration.http://www.wired.com/science/planetearth...
A diesel engine providing electricity for an electric drive can operate at its most efficient optimal RPM. It can be turned off when the vehicle is stopped and the vehicle can start on battery power to the electric motors, The series hybrid configuration can eliminate the need for a transmission. Finally regenerative breaking is a very easy adaptation in such a hybrid vehicle. And so while the engine configuration may be slightly less efficient the overall vehicle efficiency can improve substantially. Fuel economy for hybrid vehicles can be more than double that for petrochemical alone counterparts. Much depends upon how the hybrid technology is implemented. Not all hybrids are equal.
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The battery electric vehicle has an efficiency around 90% with some electric motors in highly efficient solar cars close to 99% efficient. A diesel engine does not have sufficient low end torque for starting a vehicle without the parasitic transmission. An electric car does not need a transmission. as it has full torque at 0 RPM. An idling diesel engine in a vehicle stopped in traffic is doing no useful work and its efficiency is 0. An electric vehicle stopped in traffic uses no energy for its motor.
We can calculate the efficiency of a diesel generator by comparing the BTU content of the fuel and the BTU content of the electricity produced. Gensets listed here fall in the 20% to 30 % range. http://generators.findthebest.com/app-qu...

OK, but OP seemed to be asking about a diesel generator for electric power supply. I doubt he has vehicle transmission in mind.