Perhaps the alternator was rated at a theoretical maximum of 6 kW. In typical marketing BS they failed to point out the engine was incapable of driving the alternator to its full potential.

Could be a mistake or ratings confusion.

Motor ratings aren't fixed limits. If the seller is playing the game 6.5HP means the engine is capable of outputting nearly 5kW continuously, which a 6kW alternator will handle comfortably, supporting a load of about 4.4kW without bother.

But say the load includes a deep-freeze which comes on intermittently every hour or so and demands an extra 1.5kW for 5 minutes. Will the extra demand stall the engine because it has to deliver more than 6.5HP?

Probably not: most engines can exceed their ratings for some time. This happens in a car, when the driver drops a gear and floors the accelerator whilst overtaking. The engine sees an easy load allowing the pistons to move faster and consume more fuel. It's power output at high RPM will be considerably above the manufacturers rating, which describes the engines ability to deliver power continuously under ordinary driving conditions.

I think the generator engine would do the same, providing an extra couple of HP for some time. The awkward question is how long? The answer depends on conservative the seller is. He might assume users will only use the generator infrequently, and his inexpensive machine will last beyond the guarantee even if abused. Or he might be selling a top-end generator, expected to do solid hard work for several years. An owner buying a generator to cover occasional power-cuts probably doesn't need to spend big money on the long-lasting generator.

A mistake is more worrying – the generator might not be what it seems at all! And those numbers are strange…

Dave

Edited By SillyOldDuffer on 22/08/2022 19:42:06

Electrical power tends to be the same irrespective of ambient pressure or temperature.

Power for I C Engines is usually quoted under standard conditions (Often 20'C and 750 mm Hg. There are BS, ASA and ISO standards ), so on a cold winter day, possibly with high barometric pressure, the engine might be capable of delivering more than its claimed "6 hp"

On a hot day with low atmospheric pressure it won't!

Which is why many super or turbocharged engines use intercoolers to increase the density of the air going into the cylinders.

And this ignores any variation in the calorific value of the fuel and it's temperature, or local humidity!

(We always rated our engines at 40'C fuel inlet to the injection pump. A competitor, in one field, used 20'C for their power claims. On one engine this amounted to a 18 bhp difference!

Many years previously, one engine used an injection pump, known for churning the fuel and heating it. As the engine ran, you could watch the load carried by the dynamometer decreasing as the fuel temperature increased. Consequently, we only measured power output at a set temperature. )

Howard

How to get more power out of an IC engine deserves it's own thread! There's a lot to say on the subject.

But I stand by my assertion that the single figure HP rating of an engine should be it's average continuous output, not the maximum it's capable of.

Back to basics, one horsepower is the average amount of steady protracted work that can be delivered by an average horse. I recall it was first defined formally by James Watt measuring the amount of water lifted a known height by a team of horses working a bucket pump powered by a horse gin in a brewery. (The horses walked continuously in a circle to drive a windlass.)

An average horse can deliver about 750W steadily between meal breaks. A fit man delivers about 200W in the same conditions. Both horse and man can do more work in short bursts, easily exceeding the average. A trained athlete can output about 1.5kW, but not for long. IC engines also have power curves, but the quoted rating should be the power the engine can deliver continuously without overheating or causing excessive wear and tear. 'Should be' because we all know about salesmen!

Vehicle dynamometers don't tell the whole story – for example, they chart an engine's power curve for a particular load, and that load is unlikely to show what the engine would output flat-out in second-gear hauling a heavy caravan up a steep hill. (By a driver who ignored the steam, temperature gauge, noise, and smell!)

Modern cars are 'quite interesting' because they're fitted with engine management units that override what the driver does. My eco town car's engine is tuned for good fuel economy and nothing I do at the controls makes it sporty. There is a sporty model of the same car with much the same engine, except the EMU is set for performance rather than economy. Unless you know how to reprogram an EMU it's easier to overdrive mechanically managed engines because they're less governed.

Fundamentally the maximum power output of an IC engine is limited by the amount of energy that can be extracted from the cylinders in a given time. That suggests several ways of improving power output:

• Run at higher RPM
• Optimise ignition timing for power output (rather than torque, economy or emissions)
• Increase the volume of the cylinders, diameter and depth
• Increase compression
• Improve fluid flow through valves, exhaust and intake
• Improve fuel/air mixing with better carburation or injection (not easy)
• Optimise fuel/air proportions to match current operating conditions. (lean and rich mixtures)
• Increase fuel density in the cylinder by blowing the intake with a supercharger or turbocharger
• Increase fuel density by cooling the intake with an intercooler after the blower
• Increase fuel density by injecting methanol or water into the mix
• Increase energy density by replacing air with Oxygen
• Increase heat value of the fuel (exotic, because oil is almost as good as it gets.)

Nitrous Oxide works well because it combines 3 effects. Inside the cylinder, N2O adds a useful amount of extra heat during the burn, but it's main benefit is to chill the fuel/air mix going in so a greater weight of it ends up in the cylinder. Another important benefit is that the small amount that can be carried means it can only be applied to the engine in short bursts. As the boost doesn't last long enough to seriously overstress the engine and it's cooling system, Nitrous Oxide can be fitted without needing other parts of the engine to be uprated

The downside of these techniques is the engine takes a thrashing that reduces it's useful life. Formula 1 engines only last one race. Balancing performance against reliability is tricky.

Anyone on the forum deep into souping-up cars? I'm interested in the theory, but have no practical experience.

Dave

Edited By SillyOldDuffer on 23/08/2022 11:00:57

If you're going to buy something along these lines check it out on youtube first

There's an army of guys making youtube vids about them

taking them to bits, wiring up load meters/oscilliscopes etc

The standard rating an engine is at Full Load Rated Speed.

If it says "6 hp" that what bit should deliver over a long period, (Thousands of hours not hundreds ) unless it mis a tiny engine ( <250 cc ? ) being pushed right to the limit

For people like Enzo Ferrari and Colin Chapman, it was enough for an engine to survive just long enough to win the race.

For mere mortals like us, the engine that we use, output is traded for life. A petrol engine producing 259 bho per litre is possible, but it won't last as long as an otherwise identical one producing 50 bhp per litre.

We hope that our car engines will run reliably for 100,000 miles or more. So we are looking for a rating that is more or less continuous, even if we use maximum power for a very small percentage of the time..

No point in having a machine that spends more time being repaired or tuned than being used.

Nigel Mansell said that that was why he sold his Ferrari.

Also a short life racing engine will not be as tractable as one with less extreme valve timings.

The Sabre Marathon engine for power boat racing was a diesel giving 100 bhp per litre. Tractable? No way!, Would not even start unless the jacker water was 70'C or above. Open the throttle too quickly and the boat would come off the plane and sink into the water under a dense black cloud! But when it was on song!!!!!!!!!!!

The early BMW engines tuned for use in Frazer Nash cars were, for their time, powerful, but torque tended to increase quite suddenly as the engine speed increased. This made the car's handling quite difficult..

It also meant that faced with a hill, it was no good expecting the engine to slog at low speeds.

Scavenge blown two strokes tend be the same. It was not for nothing that Foden FD6 powered lorries had 12 gear ratios available. Little low speed torque, so had to kept spinning fast…

Torque back up is unimportant in a genset; it is a constant speed engine. An alternator gen set should be governed so that the speed varies by 5% or less between full and no load, (Absolutely necessary if expected to run in parallel with one or more other gensets, or an infinite bus, aka the mains )

Who wants anything frequency sensitive, such a mains fed electric clock or induction motor, running at varying speeds?

If "mains" frequency is wrong, fluorescent tubes might be more difficult to "strike" since they rely on resonance to produce the spike for starting.

For multiple sets, one was usually chosen as the Master and fitted with an isochronous governor, where the speed did change with variations of load, and the slave, droop governed, sets were paralleled with it.

Without such a Master set, trying to keep two or more alternator sets in sync is almost impossible.

We had a genset back, that had powered fluorescent lights during construction of the channel tunnel. It had covered over 27,000 hours, and was good for more. Ideal conditions, constant speed and load with strictly observed maintenance schedules. TML demanded it back after it had been stripped and inspected!

Diesel electric locomotives tend to run the engine at more or less constant speed controlling output by varying the field on the alternator.

A DC genset needs to run at more or less constant speed, or output voltage varies.

But don't expect too precise, let alone isochronous, governing, or an extremely long life on a "Built to a low price" genset.

You get what you pay for!

Howard

Nitrous oxide increases the amount of oxygen in the intake charge, allowing more fuel to be burned. Simple as that. Gains from increasing the density of the intake charge by cooling are minimal.