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

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

(OP)
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)?

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

Those generator engines are governed to a maximum of 3600 RPMs. Do you know how horsepower is calculated? They don't turbocharge them because they are carbureted, have to meet emissions standards, be reliable, and they are air cooled.

Why are cars so backward in terms of technology? I have a yz250 2 cycle motocross bike that makes just shy of 50 horsepower at 10,000 RPMs. By that standard, my 5.3L chevy Silverado should have over 900 horsepower.

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

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

Engine speed is a factor - to produce 50Hz with a 2-pole generator requires the engine turn at 3000 rpm, and for 60hz at 3600 rpm. Not great for maximising power output from a small engine. The inverter type run at higher speeds to make better use of the engine capability, but the electronically synthesised output of inverter generators can cause problems with certain loads, notably motors.

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

I have a Yamaha EF2000i. I don't know what the displacement of the engine is or what it revs to, but it does have a switch for enabling or disabling "eco" mode (inverter operation) and when it is operating under load and you flip that switch, there is little or no change in engine speed. That would suggest engine speed somewhere near 3600 rpm at rated power, and the sound of the engine is consistent with that. I also have a 125cc 4-stroke motorcycle (12 horsepower) and it's obvious that the generator engine revs nowhere near as high as the bike engine does. If the 125cc engine that makes 12 hp at around 10,000 rpm makes the same torque at a third of those revs (which would be roughly the case if the cam timing were optimized for the lower revs) it would be making around 4 hp i.e. 3 kW, which is somewhere near what it would have to do in order to operate that generator.

The low operating revs is also in the interest of having the engine last a long time. These engines have to be capable of operating at rated power continuously in cold weather or hot weather or anything in between. They're also expected to start with the pull cord and go immediately to rated power without this causing problems. This is not an application for a high-strung, highly-stressed engine. It's an application for a mildly-tuned understressed engine ... and that's what it is.

Turbochargers add cost, complexity, make the engine higher stressed, would probably want liquid cooling, and they don't scale down well to tiny engine sizes, all of which are unfavorable for this application.

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

(OP)
Panther140 - These small generators have no catalytic converter that I can see (certainly each one I've disassembled has had only a muffler as the sole exhaust component). So your emissions argument is moot because the ones on sale here pollute much more than even the dirtiest car made in the last few decades. You can even smell hydrocarbons in the exhaust of one of these generators. Secondly, it is possible to turbocharge a carburetted engine. But why stop there? Why not use fuel injectors and create the most technologically-advanced high-performing generator on the market?

Scotty - the generators I mentioned all use inverters so engine RPM is not the only issue. In fact the Honda inverter units I mentioned run at 5,000rpm at full load.

Brian - from what I know of inverter generators, they need not run at 3000 or 3600rpm, as the output frequency is synthesized electronically regardless of the position of the eco switch. Here's a quote from the Honda website -

Quote (Honda)

"First, the generator's alternator produces high voltage multiphase AC power. The AC power is then converted to DC. Finally the DC power is converted back to AC by the inverter. The inverter also smoothes and cleans the power to make it high quality. A special microprocessor controls the entire process, as well as the speed of the engine."
Since there's a DC step, it would be of no advantage to rev the engine to 3000 or 3600 rpm exactly, so logically I would expect that what you hear is it revving to close to peak power rpm (which it would be revving at in ECO mode anyway if it's close to full load). If it's anything like the Honda EU1000i the rpm is probably closer to 5000 than 3600. The rest of your post I agree with - and in fact suspected as much myself.

P.S. A traditional generator works nothing like an inverter one. The ones I've disassembled have had a centrifugal governer linked to the throttle to keep engine RPM fixed at 3000rpm (thereby ensuring 50Hz output), and a simple regulator that adjusts the current in the field windings of the rotor to keep the output voltage of the stator coils constant at around 240V per phase.

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

Quote (LMF5000)

...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...

I estimate that a generator made along the lines you suggest would cost perhaps 10 times as much as a common 5 kW portable unit.
There is probably little or perhaps no market for same, except maybe for government buyers where money is no object.

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

It's quite possible that it still operates as an inverter in "eco" mode, freeing the engine from the fixed RPM relationship.

Still, this doesn't change anything. The engine as it is now is a simple single cylinder 4 stroke air cooled engine with an overhead camshaft and 2 valves. Why add a ton of complexity just to be able to make the engine a little bit smaller (while adding a bunch of extra parts that take up even MORE space and add even MORE weight - and more cost)?

Just because an engine doesn't have a turbocharger doesn't make it "less efficient" in some way. Our family's experience with turbocharged gasoline engines in automotive applications has been that they can be more economical in specific situations (light load when you are NOT using the turbocharger!) but if you just drive it without paying attention to economy, or (worse) if you actually put it under load so that it is running under boost, they use MORE fuel than a bigger but more mildly-tuned non-turbo engine would use.

The generator engines don't have catalysts because the EPA (or EU) evidently hasn't gotten after them yet. Lawnmowers don't have catalysts, either. My 12 hp 125cc motorcycle has closed-loop EFI with 3-way catalyst.

Honda is a large company. So is Yamaha. Both companies make millions of small engines every year. I think you can rest assured that both companies have done the necessary research to find the right balance between performance, weight, cost, emissions, efficiency, noise, and so forth. Why overcomplexify something that doesn't need it?

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

(OP)
Wayne - why is that? Mass production would drive the prices down. In my country the 1 liter 3 cylinder turbo ford fiesta ecoboost is only 10% more expensive than the 1.25 liter 4 cylinder non-turbo ford fiesta. A company like Honda can also use the same engines in their outboard motors, water pumps etc.

Brian - some interesting points. I appreciate that turbos are slightly less efficient than NA at many operating points, but I was not aiming to optimise for fuel consumption (g per kWh) - rather, I am trying to optimise for weight and size. If I want to run a 2kW toaster plus a 2kW aircon, why should I need close to 50kg worth of generator?

Your line about the research the big companies do was true when they introduced the models... but I remember looking at the Honda portables lineup about a decade ago and nothing's changed. They have the same 1kw and 2kw models they had years ago. I'm inclined to think the market doesn't exist because no manufacturer has been bold enough to introduce the equipment. I mean, before Apple came along there was no market for €700 phones either (unless you count Microsoft's lineup of PDAs). Do you think a construction worker would choose to wheel around a 50kg 5kw unit that must be transported by truck, if he had the option of paying 10% or 25% more for a 15kg unit that can do the same task and can fit in a hatchback? Honda generators are already twice the price of every other brand in my country, and the only thing going for them is the perceived quality - because the specs are no better than the Kipor or Yamaha competition on paper. So why not a "premium" model where the extra cash buys better performance?

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

2
These tiny generator manufacturers have two options: sell as cheap as possible, or stake a claim on a premium reputation.

Selling as cheap as possible means cut cost wherever possible.

Establishing a premium reputation with a premium price means your customers have to perceive it as reliable. However, these same customers will horribly abuse their units, never adjusting valves, leaving it out in the rain, running on the same oil until it is a puddle of syrupy sludge, leaving old gas in it for years and expecting it to start again, full of water and bugs, running with inadequate cooling, etc. etc. etc.

The one premium manufacturer (Honda) seems to deal with these things remarkably well. To that end, I would expect them to:

1. Be very conservative in their design changes, because any change can have far reaching unintended consequences
2. Keep all parts running inside of conservative design standards because of how abused they know they will be.

Your hot-rod generator would run well for the 1% of people who actually maintain their units, but the other 99% would just perceive it as a "piece of junk" because something would always be going wrong with it, from their uneducated perspective.

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

A conventional style 5kW generator can be had for under $500, compared to Honda's 3kW inverter unit for about $2400. I think that adding your proposed requirements "...3-5kW unit that can be easily carried with one hand..." would drive the price up even more; perhaps not to $5000, but certainly by a significant amount. My guess is that paring the Honda down from 78 pounds to half that with no reduction in output could double its cost, and that getting 5kW out of it would almost certainly do so.

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

2
When you are talking about small engines, don't confuse specs with performance.
When I was teaching, we shared a lunch room with the small engine repair instructor.
Whenever the HP of small engines came up, he would say;
"The first liar doesn't stand a chance."
You can rate the HP of a small engine under optimum conditions, at the sweet spot RPM using best quality fuel.
BUT, if you are building an small engine to power a generator you may want to consider that it may not be running at the optimum RPM, it may not have the best fuel, and it will still be expected to produce rated power after a few years and/or a few thousand hours.
I am sure that if an engine rated for genset use was marketed as a stand alone, general purpose engine it could have much better specs.
The actual HP of a small engine is not often checked by the owner. It may be difficult to check the HP of a small general purpose engine.
It is often very easy to see if a genset is putting out full rated power.

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

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

Quote (LMF5000 (Mechanical) )

LMF5000 (Mechanical)
(OP)
16 Feb 17 22:16
Panther140 - These small generators have no catalytic converter that I can see (certainly each one I've disassembled has had only a muffler as the sole exhaust component). So your emissions argument is moot because the ones on sale here pollute much more than even the dirtiest car made in the last few decades. You can even smell hydrocarbons in the exhaust of one of these generators. Secondly, it is possible to turbocharge a carburetted engine. But why stop there? Why not use fuel injectors and create the most technologically-advanced high-performing generator on the market?

If you can find a supply of free catalytic converters and turbochargers and EFI systems that will work with those small engines and their packaging demands, you would have a point. By the way, the emissions point is not moot. They have to pass emissions standards that are set for that type of engine. Not the same standards that cars go by.

Also, they can't use catalytic converters with carbureted engines because they will clog.

Your ideas are not profitable.

Small, cheap, simple (reliable), durable.

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

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

Quote (LMF5000 (Mechanical) )


Wayne - why is that? Mass production would drive the prices down. In my country the 1 liter 3 cylinder turbo ford fiesta ecoboost is only 10% more expensive than the 1.25 liter 4 cylinder non-turbo ford fiesta. A company like Honda can also use the same engines in their outboard motors, water pumps etc.

The engine in a car is less influential to the overall cost to produce the unit than the engine is on a generator. They have less demanding packaging constraints. They can sacrifice a cylinder to gain room. The engine has a lower duty cycle. The engine in a fiesta is not designed primarily for durability like an engine in a generator is.

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

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

You can afford to many of these things when you get to 20-30kW, whole house standby units.
Some of these are turbo-diesels, many are NA nat. gas engines. They have good EMC and often elaborate muffler/Cat systems.
Of course these are $8-15,000 units so you can afford to do it.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

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

Now I'm looking forward to the LMF5000 series of hand portable generators. They won't be on the market long before Honda buys him out for hundreds of millions to shelve the designs.

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

(OP)
JohnMcNutt - great post. I think you really hit the nail on the head. I have encountered several of those kinds of people. They're the kind that boast that Toyota are reliable because they haven't changed the oil in theirs for 8 years and it still runs (albeit you can see the owner's fingerprints in the soot the exhaust leaves all over the boot). To be honest I haven't had too many problems with the cheap no-name Chinese units (I recently took apart one unit named "silented performances" - it was basically a mechanically governed Honda engine clone with a generic Briggs & Stratton style 3-phase generator). I guess a turbo would be much more sensitive to good maintenance. Though with this being a premium product we could afford to put in a proper display and an oil-quality sensor that explicitly tells the owner "oil change needed" sufficiently in advance of the point where engine damage starts to happen.

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

There are already much better overall quality units on the market. Honda (and to a lesser extent Yamaha) definitely trade on name, but seem to have hit that sector of the market where additional cost is justified, but the overall size of the platform is still quite compact.

The (smaller) Honda units are great for camping and so on, as they can be relocated easily.

If more reliability, plus less noise and maintenance is needed, there are plenty of small diesel units around (the Hatz options at least, come with integrated silent packs) that offer reasonable performance and economy. But they're much heavier, and more expensive than the equivalent small petrol ones.

The most pertinent condition really is the amount of use it gets. Its hard to justify $5k for a Hatz Diesel set for a 4 hour outage every 3 years, at least for domestic use. $500 for a petrol generator that requires a bit more effort to maintain for the same conditions is much more appealing for a home owner.

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

(OP)
3DDave - haha. I've been daydreaming about that for ages. I will create the iPhone equivalent of generators. Here's how I think the engine would look:
- 2-cylinder horizontally opposed layout [gives perfect primary and secondary balance without all the dead weight a single-cyl has on the crankshaft for vibration reduction]
- 2-stroke Diesel [no valvetrain to add weight and maintenance requirements; also double power output per revolution vs. 4-stroke. Diesel for efficiency]
- turbocharged [increases volumetric efficiency and can't have a 2-stroke diesel without one]
- liquid cooled and electronically fuel-injected [for performance and dependability]
- starting system powered by a lithium polymer battery [1/10th the weight of lead-acid for same starting current. Maybe use clever electronics to do away with a separate starter motor and use the generator itself to spin up the engine like they do in some turbofan engines (Learjet 60 engines come to mind)]
- alternator direct-driven by engine to do away with flywheel [like Honda inverter units]

On the electrical side, here's what I imagine:
- inverter driven output with all the usual protection features
- supercapacitors in the DC stage [to greatly improve surge current capacity for starting large inductive loads like air conditioners]
- engine rpm kept as low as possible - just enough to drive the load (sudden demand spikes supplied by supercaps while engine catches up, instead of relying on drivetrain momentum)

Finally, I'd put in a large dot-matrix display that gives useful info like:
- output voltage, frequency, current and power
- engine rpm, coolant temp and load level (in percentage of max power output)
- fuel qty remaining, fuel flow rate and projected endurance (in hours, at current flow rate)
- explicit messages in case of any failures (eg. "Oil qty low")
- maybe add a wireless module so you can view all this info from your cabin and start and stop the unit remotely.

Desired specs: 10-20kg, 7-10kW, <70dB @7m, fits inside a cube of side 500mm, 5 hours endurance, -5 to +50 deg. C operating environment, cost below $5000.

Granted it would have to be machined from a billet of unobtanium, but would be nice to have one :D

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

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

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

Quote (OP)

- turbocharged [increases volumetric efficiency and can't have a 2-stroke diesel without one]
GM's Detroit Diesel Division. brought out the original 71 Series two cycle engine in 1938.
The original "Screamin' Jimmy"
Eventually they produced the 53 Series, the 71 Series, the 92 Series, the 110 Series and the 149 Series.
Also the 51 Series, 54 Cu/in per cyl. introduced in '51. This was based on the 53 Series engine but did not have any valves. Both the intake and exhaust were ported.
Probably several millions of these were Naturally Aspirated.
The 6V-92TT introduced in 1974 may have been the first turbo charged Detroit Diesel. This may not have been available in a naturally aspirtated model.
All other Series were available without the turbo.
The engines did have a low pressure scavenging blower to flush out the exhaust gasses, not to increase the charge volume.
The famous 6-71 blower does not become a super-charger until it is removed from the diesel engine and installed on a race car.
Eventually the turbo became an option on many of the Jimmy diesels but the scavenging blower remained in place when the turbo was used.

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

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

(OP)

Quote (wayne440)

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 tongue.

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

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

2
The average outboard has a huge cooling system that's not particularly portable.

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

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

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

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.)

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

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.

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

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_elements/resources/9... ). 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/solutions-alpha-cata... or the 1.5kVA inverters for up to 24kVA of the AMPS24 inverter systems http://www.broadband.alpha.com/products/item/amps2... 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/whitepaper...



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

(OP)
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.

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

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.

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

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.

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

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.

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

These folks scale down pretty far - 30 kW.

Capstone Turbines

old field guy

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

(OP)
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

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

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

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

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. winky smile

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

(OP)
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?

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

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.

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

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

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

(OP)
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!

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

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

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

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

Reminds me of the old watch joke:

Quote:

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!'

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

Quote (Wayne440)

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

I also suspect a micro-scale turbine powered generator that needs engine teardown and rebuilding every 25 hours would not be great for anyone.

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