Why do modern small petrol generators (~1kW) have such mediocre specs? 11

[LMF5000]

I refer to the tiny suitcase-style generators with plastic cases that you take with you on barbecues, like the Honda EU1000i and all its competitors from Yamaha, Kipor, Hyundai and so on.

In general, they seem to have single-cylinder 4-stroke engines of 50-100cc, weigh 10-20kg, make 60-90 dB of noise, cost €400-1000 and output just 700-1100 watts.

In contrast, a modern-day naturally-aspirated car engine is hitting 100 bhp per liter. A 100 bhp engine weighs around 70kg (including gearbox), can make over 800W while idling near-silently (calculated from 70A alternator output * 12V), and costs maybe €5000 new (engine alone).

By those specs, a 50cc generator engine should be able to make around 5bhp (3.7 kW), and assuming 90% conversion efficiency should easily reach 3kW sustained output at line voltage.

Why are generators still so backward in terms of technology? Why has no manufacturer created a 3-5kW unit that can be easily carried with one hand (say by using a very small petrol engine and adding a small lightweight turbocharger (as they are doing with European cars like the Fiesta Ecoboost these days in an attempt to save fuel)?

 

[LMF5000]

If you bring that to market at $5K or less, I'll buy the first one.

Wow, things are moving along! Already my first launch customer .

Joking aside, how much would you envisage such a unit would cost? Keeping in mind that the average 10hp outboard motor already has about half the components needed (sans inverter and radiator), and sells for $2600

 

[3DDave]

The average outboard has a huge cooling system that's not particularly portable.

 

[Panther140]

What do you mean by "5 hours endurance"?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[BrianPetersen]

I think you will encounter scale-down difficulties with the diesel fuel injection system (too-small injection quantity per stroke), turbocharger (too small), plus all the piston ring lubrication difficulties that Detroit Diesel went through. (If the piston rings get enough oil, the engine has excessive lube oil consumption and high emissions. If the piston rings don't have enough oil, they don't last.)

 

[wayne440]

A typical 10 horsepower outboard motor is around 200cc displacement and runs on gasoline- that is a long way from a 20kg 2 stroke diesel. The closest contemporary set to what you envision is probably the Honda EU7000i, which can be had for about $4K.

Displacement 389cc
AC Output 120/240V 7000W max. (58.3/29.1A) 5500W rated (45.8/22.9A)
Starting System Recoil, electric
Fuel Tank Capacity 5.1 gal.
Run Time per Tankful 6.5 hrs.@ rated load, 18.0 hrs. @ 1/4 load
Dimensions (L x W x H) 33.4" x 27.6" x 28.4"
Noise Level 60 dB(A) @ rated load, 52 dB(A) @ 1/4 load
Dry Weight 261 lbs.

That is a lot heavier (about 118kg) and bigger than your target, plus the challenge of making it
diesel powered. Frankly, I'd be amazed if the unit you describe can be made and sold in
quantity for under $10k.

 

[DanEE]

supercapacitors in the DC stage [to greatly improve surge current capacity for starting large inductive loads like air conditioners]


Don't forget the cost of approx 4x transistor, magnetics and everything else in the inverter to present a sufficiently low source impedance needed to produce and withstand the peak current capacity (4x in the neighborhood for typical locked rotor current requirement to start many motors).

For example, I have a 600 watt (combined output of the 2 channels into 4 ohm loads) audio power amp, DC coupled to also be used as a DC servo power amp) that was legendary in its day for being bulletproof (Crown DC-300a partial specs here

http://adn.harmanpro.com/site_eleme...300A-Instruction-Manual-dc300aim_original.pdf

). Has 1kW power supply with large caps for stored energy and 2400 watts of transistor power output capability to provide a peak 120 amp capability, 28 amp maximum steady state from the final output stage to make it stay together while handling peak currents and fly back potentials. Extensive VI limiting built in so it stays together when pushed beyond the limit.

While the audio/servo amp is a different application/hardware territory, my recollection from electro mechanical energy conversion courses is that the issues involved whether dealing with linear actuator systems (aka audio or electro mechanical linear motion servo systems) or rotational electro mechanical systems (aka motors) are pretty much the same. The partial specs are quoted above mainly because it rare for the product designer to disclose this level of detail about the design requirements of a device with similar peak capacity to nominal output ratings, and ability to drive reactive loads with reliability.

If memory serves me well the power amp above cost about $1700 in 1978 or around $6300 today in US dollars. While technology has driven costs down somewhat, a comparable quality power amp of current generation technology is in the $7 - 9 thousand dollar range. Audio power amps of this genre are limited, not mass market technology, but as noted above your ideal system would have the same limited market/pricing issue.

The commercial UPS systems I've dealt with have a combination of very fast electronic shutdown and/or magnetic breakers to protect the very expensive transistor technology in the inverters on extreme surge overload. The individual 2.5kVA inverters using IGBT transistor technology that combine up to make up 1, 2 or 3 phase power up to 75kVA in the AMPS 80

http://www.alpha.ca/solutions/solut...ard-systems/inverter-solutions/item/amps80-hp

or the 1.5kVA inverters for up to 24kVA of the AMPS24 inverter systems

http://www.broadband.alpha.com/products/item/amps24-hp-2

are spec'd to 10x I nominal rated current for 20msec, 150% overload for 5 seconds and permanent overload at 110%.. Some pricey but beautiful state of the art hardware.

This white paper written back in 1998 touches on a couple of the items on the above. Per this paper the telecom industry has long needed better small capacity 2 - 10kW generators for backup power to keep the distributed high speed data networks in our residential neighborhoods running during power outages.

http://www.broadband.alpha.com/download/whitepapers/EngineGenOSP10-98.pdf

 

[LMF5000]

Panther - by 5 hours endurance I mean it can run 5 hours on a single full tank at full load.

Everyone else - I thought of an outboard engine as they are small, liquid-cooled, include an alternator, and relatively light and affordable in this power range. Obviously you would remove the gearbox/prop/impeller etc., add a radiator and thermostat, increase the alternator capacity, add an inverter and throttle control etc.

Some very good responses here. I am starting to realize that this might not really be possible with a Diesel.

Would it be possible with a tiny turboshaft engine driving the generator? Similar in concept to an aircraft APU but much much smaller (say, the size of an automotive turbocharger), and with no bleed air output (all energy dedicated to shaft horsepower) and ideally jetted to burn Diesel or gasoline. The FADEC would take care of everything so all the owner would see is a single off/on/start switch to operate the device.

Advantages I see are very small size and weight for the power, very few moving parts, no dedicated cooling system needed (except for oil cooler perhaps). Disadvantages would be noise and exhaust heat management, the reliance on electricity for starting (no pull start), possibly very high fuel consumption per kWh.

 

[BrianPetersen]

Turbomachinery doesn't scale down very well. The auto industry tinkered with them in the 1960s but they were thirsty even by the standards of the day. Gas turbines work great at a utility power generation scale or a commercial aircraft propulsion scale. Turbochargers work because they have a great surplus of exhaust energy to work with and aren't called upon to deliver any shaft output.

The efficiency of a gas turbine depends on its pressure ratio just like the efficiency of a piston engine depends on its compression ratio. Getting a high pressure ratio requires using multiple stages of compression and expansion. No problem for a utility power plant. Jet aircraft use multiple turbines in the compression stage. Doing that on the scale you are talking about ... won't be practical or cost effective.

The Brayton cycle with multiple compression stages can benefit from intercooling between the compression stages. Utility power plant, no problem. Small engine ... not practical.

 

[wayne440]

Several months ago, I was part of a similar discussion wherein the intent was to produce a 100 pound, 80kW set with multi-fuel (diesel, JP-8, kerosene, gasoline) capability. I told the gentleman that if he produced a 100 pound, 20kw multi-fuel capable set at a reasonable price, I would buy the first 100 units. Since the smallest diesel powered set I know of weighs over 150 pounds and makes 2kW, I don't expect to be buying soon.

 

[LittleInch]

Think these guys are ahead of you...

http://www.bladonjets.com/

Not portable or cheap I suspect, but ready and waiting.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[oldfieldguy]

These folks scale down pretty far - 30 kW.

Capstone Turbines



old field guy

 

[LMF5000]

LittleInch - that's encouraging! If a unit is comercially available it looks like I'm not completely missing the mark! Now if someone could scale it down to suitcase size...

I don't think it's impossible. Lots of online videos showing automotive turbos converted to turbojet engines (some with actual, usable thrust). Imagine what could be done with a purpose-designed part.

Every small aircraft APU I've come across has had a single-stage centrifugal compressor and single-stage centrifugal turbine or one or more axial turbine stages. They output on the order of 30-60kVA at 115VAC (400Hz) and are about the size of a 7kW Diesel reciprocating generator and can be carried by two mechanics. They also output significant amounts of of power pneumatically in the form of high-pressure bleed air (to run the air conditioning packs and the engine starters) so definitely oversized for the shaft load alone.

It would be nice to see a multi-stage turboshaft engine designed specifically for generator applications. A (very) small automotive turbo normally costs €750 so the turbomachinery components of my mythical generator won't be prohibitively expensive.

A great starting point would be the turbine engines currently made for RC aircraft. Here's one that weighs 3.5 kg, makes 10kW of shaft horsepower, costs almost $10k, and will empty a 5-liter fuel tank in about half an hour (ouch):

https://jakadofsky.com/htm_en/daten.htm

 

[MikeHalloran]

The Capstone turbine units are surprisingly complex.
The turbine runs at ~25,000 to ~90,000 rpm, rides on air bearings, and drives the generator rotor directly.
Because the turbine generator can't change speed instantly, there is a substantial battery bank to hold up the AC output when demand steps up, and to absorb the generator output when demand steps down.
Plus a crapload of power electronics to manage all that.
It also has a recuperator, which adds considerably to its efficiency,

... making its fuel consumption only 20...25 pct worse than a similarly sized Diesel generator.



Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

The 400Hz of an aircraft generator, rather than the 50Hz or 60Hz of a terrestrial generator, is hugely significant to the amount of iron required in the stator core, and thus to the overall mass of the machine. The cross sectional area of the core iron in the flux-carrying paths of a 400Hz alternator will be 1/8 the area of an equivalent 50Hz alternator.

I can't imagine any 50Hz 60kVA generator that can be carried by two mechanics, not unless one of them is driving a forklift truck. ;-)

 

[LMF5000]

Scotty - I know aircraft systems runn at 400Hz to save on mass and size. Now considering my miracle inverter will be using an "inverter" or other kind of electronic conversion circuitry - is there an efficient way to transform 400Hz into 50Hz or 60Hz? Or does 400Hz just need as much conditioning as straight DC from an alternator-and-rectifier combo?

 

[ScottyUK]

It would need a rectifier at the front end and some DC link capacitance, although nowhere near as big as at 50Hz/60Hz. If you can feed it 3-phase 400Hz rather than 1-phase then the demands on the link capacitor are much smaller. Probably not much difference between a straight 400Hz input and using alternator input, although I'll qualify that comment as being slightly subjective without knowing the alternator speed and pole count.

 

[waross]

The rough relationship is that the weight of the iron is inversely proportional to the frequency.
Use an automotive style alternator.
It will be fast and light.
A 60 Amp automotive alternator can probably be run up to 120 Volts. That's 7200 Watts in a nice small package.
An automotive style alternator starts charging at quite a low speed.
At higher speeds it are easily capable of 100+ Volts.
Fairly high frequency but the frequency doesn't matter if you are going to rectify and invert.
The field and thus the voltage is controllable on an automotive style generator.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LMF5000]

So we take an R/C turboshaft engine plus gearbox, couple it to an automotive alternator, use the output to feed a pure sine-wave inverter, and we have a 7kW generator for under 20 kilos. Mission accomplished!

 

[wayne440]

Except for the "... <70dB @7m, fits inside a cube of side 500mm, 5 hours endurance..." portion.

 

[LMF5000]

wayne - all we need is an exhaust system and a fuel tank many times the size of the unit .

Reminds me of the old watch joke:

A chap walks into a pub carrying two suitcases. He puts them down and orders a drink. As he pays the barman notices that he is wearing a large watch.

'That's a large watch' says the barman (see I told you!)

'This watch,' says the man, 'is the very latest in high tech gadgetry. It'll tell you the time anywhere in the world. It's got GPS positioning, sat nav, and night vision. You can surf the net, make and receive telephone calls and it's got TV built in. That's Sky, Terrestrial and Sultana Sport by the way, and you can download the latest movies which it projects through a micro projector onto any flat surface.'

'That's amazing says the barman, how much did that set you back?'

'50 quid.'

'50 Quid?! You're joking!'

'No, in fact, I'll sell it to you for $55 if you like.'

'Done!' says the barman and hands over the cash. The bloke hands over the watch and the barman straps it on and starts admiring it and playing with the buttons. The man finishes his drink and goes to leave.

'Mate' says the barman, 'you've forgotten your suitcases.'

'Suitcases? Oh no, they're the batteries!'