Car Engine vs Generator

[sssutututu]

Is there a reason why generators are so extremely inefficient vs a car engine?
We ran a rebuilt P5 VPS F-P5 Skyactiv 1.5 Petrol engine at idle for a 25 hour stability test. It only consumed 11 liters @ 7.4KW on the bench. If you ran this for 100 hours connected to an inverting unit for 5-7KW you would use 36.3 Liters of 95o and have enough power for a lot of stuff at home?

What made me question this is that my generator at home uses ~10 liters for 2-3KW per 7 hours. It would consume 140 liters per 100 hours.

This used and repaired car engine is 10 times cheaper than a diesel generator that can match its efficiency.
So what gives in the generator R&D, Why is it so overwhelmingly bad? Is it made inefficient on purpose?

 

[WAslayer]

Is there a reason why generators are so extremely inefficient vs a car engine?
We ran a rebuilt P5 VPS F-P5 Skyactiv 1.5 Petrol engine at idle for a 25 hour stability test. It only consumed 11 liters @ 7.4KW on the bench. If you ran this for 100 hours connected to an inverting unit for 5-7KW you would use 36.3 Liters of 95o and have enough power for a lot of stuff at home?

What made me question this is that my generator at home uses ~10 liters for 2-3KW per 7 hours. It would consume 140 liters per 100 hours.

This used and repaired car engine is 10 times cheaper than a diesel generator that can match its efficiency.
So what gives in the generator R&D, Why is it so overwhelmingly bad? Is it made inefficient on purpose?

What load did you put on the engine..?

Generators always run under load and proper commercial generators are designed to produce a specific amount of power at a consistent load..

In short, you can't compare a car engine running under no load, to a generator which is always running under load..

Hook your car engine up in place of a generator engine and I am sure you are likely to find the generator to be more efficient..

 

[Solarion]

Is there a reason why generators are so extremely inefficient vs a car engine?
We ran a rebuilt P5 VPS F-P5 Skyactiv 1.5 Petrol engine at idle for a 25 hour stability test. It only consumed 11 liters @ 7.4KW on the bench. If you ran this for 100 hours connected to an inverting unit for 5-7KW you would use 36.3 Liters of 95o and have enough power for a lot of stuff at home?

What made me question this is that my generator at home uses ~10 liters for 2-3KW per 7 hours. It would consume 140 liters per 100 hours.

This used and repaired car engine is 10 times cheaper than a diesel generator that can match its efficiency.
So what gives in the generator R&D, Why is it so overwhelmingly bad? Is it made inefficient on purpose?

Propably something to do with torque. The magic is in the crankshaft. A gene drives the load whilst a car engine swings with its own inertia. Just my two cents. No expert on all things engineering.

 

[B-1]

I'm not sure what the question is? A modern car engine is very expensive to manufacture and far more complicated. If you had to buy it new including the ECU etc it would be several multiples of a generator engine. Its more efficient because of all the technology but also more expensive.

 

[TheChamp]

Interesting experiments, I have always wondered about this. I guess no one ever thought about fine tuning a generator, everyone just assume it is a small engine and it will consume less, plus the tech is just old compared to new engines.

 

[PhireSide]

The other issue is that while an engine at idle that is big enough might make enough torque, you end up with different issues like glazing of the cylinder walls and oil dilution/excess wear due to cooler running temperatures.

Your engine will also take longer to heat up, and will wear out quicker when cold as all the parts will be at loose tolerances until at operating temp. So you might end up replacing parts sooner than expected.

 

[Mannydefreitas]

Generators run at constant revs.

Car engines vary the revs.

A car engine, running, is over R100k new, you can get a Gennie for R20k.

A Gennie can stand in a room and run for hours.
Air cooled.

Car engine would need major cooling to bench run static.

My Gennie 5kVA uses about 2 liters per hour under load

A small car doing 60km/hr would use 6 liters in an hour.
Take off the weight of driver and body and it's 4 liters.

I'm no expert ~ just my rambling thoughts...

 

[Tomtomtom]

Is there a reason why generators are so extremely inefficient vs a car engine?
We ran a rebuilt P5 VPS F-P5 Skyactiv 1.5 Petrol engine at idle for a 25 hour stability test. It only consumed 11 liters @ 7.4KW on the bench.

What were you measuring? Mechanical power?

To get to electrical power you will have conversion losses.

This probably accounts for most of the difference.

I would still expect a car engine to be slightly more efficient because they are built for it, while generators typically are not.

 

[Priapus]

Interesting experiments, I have always wondered about this. I guess no one ever thought about fine tuning a generator, everyone just assume it is a small engine and it will consume less, plus the tech is just old compared to new engines.

Nope completely wrong. As others mention a generator is always under load. A car engine isn’t, at idle. That makes a big difference.

Got nothing to do with “tuning” and since generators use fuel to run and make power - efficiency is a core part of their design.

 

[NarrowBandFtw]

some companies have indeed taken car engine tech to produce modern efficient generators ... somewhat predictably to charge an EV's battery though

Mazda MX-30 R-EV Brings Back the Rotary, but Not Like You Imagined

The rotary has returned, but this time around, the 0.8-liter single-rotor gasoline engine serves as a generator to charge the battery.

www.caranddriver.com

 

[Lupus]

Generators run at constant revs.

Car engines vary the revs.

A car engine, running, is over R100k new, you can get a Gennie for R20k.

A Gennie can stand in a room and run for hours.
Air cooled.

Car engine would need major cooling to bench run static.

My Gennie 5kVA uses about 2 liters per hour under load

A small car doing 60km/hr would use 6 liters in an hour.
Take off the weight of driver and body and it's 4 liters.

I'm no expert ~ just my rambling thoughts...

6l for 60kms? That's a very inefficient car, most small cars on long trips are down to 5/100, still would be 3l though

 

[Mannydefreitas]

The BMW i3 is an EV.
The BMW i3RE (Range Extender) is an EV with a BMW 650 motorbike engine in the back to act as a Gennie to charge the Battery.

Obviously BM felt this was a back handed way of avoiding calling a Hybrid a Hybrid.

 

[Tomtomtom]

The BMW i3 is an EV.
The BMW i3RE (Range Extender) is an EV with a BMW 650 motorbike engine in the back to act as a Gennie to charge the Battery.

Obviously BM felt this was a back handed way of avoiding calling a Hybrid a Hybrid.

They don't want people to notice that this is basically the "hybrid" powertrain that freight locomotives and ships have been using for decades. Just with some batteries for buffer.

 

[PhireSide]

They don't want people to notice that this is basically the "hybrid" powertrain that freight locomotives and ships have been using for decades. Just with some batteries for buffer.

No hybrid will ever sound as cool as this though

 

[pieter_]

Is there a reason why generators are so extremely inefficient vs a car engine?
We ran a rebuilt P5 VPS F-P5 Skyactiv 1.5 Petrol engine at idle for a 25 hour stability test. It only consumed 11 liters @ 7.4KW on the bench. If you ran this for 100 hours connected to an inverting unit for 5-7KW you would use 36.3 Liters of 95o and have enough power for a lot of stuff at home?

What made me question this is that my generator at home uses ~10 liters for 2-3KW per 7 hours. It would consume 140 liters per 100 hours.

This used and repaired car engine is 10 times cheaper than a diesel generator that can match its efficiency.
So what gives in the generator R&D, Why is it so overwhelmingly bad? Is it made inefficient on purpose?

Made in China... They give you something that "works" they don't care for how long, they don't care how heavy it is on fuel. They know most people will not know how to build their own generator out of a car engine.

 

[dillinger]

With generators you are converting one form of energy to another. Fuel to mechanical energy and then to electrical energy. There are tables out there that indicate the calorific value of different fuels and then you can calculate your consumption according to your load. Think the last time I looked at the tables you need 300ml of diesel to make 1KW per hour of electrical energy. I think over all diesel generators are 14% efficient and petrol generators12%.

Diesel has a higher calorific value then petrol so diesel generators are more efficient.

The diesel engines on generators rev at a constant 1500RPM. The manufacturer will never tell you how much torque or what the torque curve is as that’s the secret behind it being able to take load and carry the load successfully.

Lets look at a small FAW diesel generator 15KVA 3 Phase (24Amp per phase), displacement 2.2L output of diesel engine 17KW (22HP), fuel consumption 5.1L per hour at full load. Manufacturer specs.

So if we want 17KW of power and the calorific value of diesel is 1KW per 0.3L

17x0.3=5.1L per hour

I always say its cheap to buy a gen but it costs a arm and leg for fuel and services.

 

[dillinger]

Is there a reason why generators are so extremely inefficient vs a car engine?
We ran a rebuilt P5 VPS F-P5 Skyactiv 1.5 Petrol engine at idle for a 25 hour stability test. It only consumed 11 liters @ 7.4KW on the bench. If you ran this for 100 hours connected to an inverting unit for 5-7KW you would use 36.3 Liters of 95o and have enough power for a lot of stuff at home?

What made me question this is that my generator at home uses ~10 liters for 2-3KW per 7 hours. It would consume 140 liters per 100 hours.

This used and repaired car engine is 10 times cheaper than a diesel generator that can match its efficiency.
So what gives in the generator R&D, Why is it so overwhelmingly bad? Is it made inefficient on purpose?

You will need a mighty big petrol engine to produce the same power and torque at 1500RPM as a diesel engine is able to deliver at 1500RPM. Diesel engine max torque is at 1500RPM and on the petrol engine it will be in the region of 5000RPM.

You can take a diesel engine and convert it to natural gas, remove injectors and fit sparkplugs, add a ignition system and a fuel injection system that works the same a carburetor, you loose 50% rating, so you start with 100KW and end with 50KW

 

[SauRoNZA]

The BMW i3 is an EV.
The BMW i3RE (Range Extender) is an EV with a BMW 650 motorbike engine in the back to act as a Gennie to charge the Battery.

Obviously BM felt this was a back handed way of avoiding calling a Hybrid a Hybrid.

Doesn’t it power a generator?

Rather than being the generator?

 

[Gnome]

You will need a mighty big petrol engine to produce the same power and torque at 1500RPM as a diesel engine is able to deliver at 1500RPM. Diesel engine max torque is at 1500RPM and on the petrol engine it will be in the region of 5000RPM.

Peak torque & peak power of different types of engines is not at all static as you make it seem here.
Long story short the peak torque & peak power are determined by how the engine is engineered.
It is much easier to build a smaller engine that revs higher due to the lower weight.
Similarly diesel engines are quite heavy and it takes a lot more work to make it rev higher, but formula 1 type diesel engines have been made that rev significantly higher than a production car engine with torque and power to match.

If you are still not convinced, Formula 1 engines rev incredible high with peak torque and power far beyond the standard revs of a car engine at practically every volume they've been produced. Similarly high revving production car engines exist that make peak torque and power far beyond 5000 RPM. But you also get petrol engines that make peak torque at around 2800 RPM (common Opel Corsa 1.6 8v engine for example). Motor cycle engines typically make most torque and power significantly higher up in the range.

There is literally no rule like you are positing here, it is a complete fantasy.

The only rule is that the higher your RPM the more power you can make, so by pushing your peak torque up in the range, the more power you get out of the given engine. And that is the very simple reason that since the invention of engines, they've been pushing the revs and torque ranges up. Including for diesel engines.

"Diesel people" always going on about lower RPM don't really understand the engineering behind engines which hasn't helped the situation.

Is there a reason why generators are so extremely inefficient vs a car engine?

They are incredible efficient.
And it goes beyond the power and torque discussion or the fact that the petrol engine you had on a dyno was unloaded.

2 Stroke engines are often used because they are more efficient. Push-rods instead of OHC because OHC is a significant power sap in car engines (valve springs sap power) and so on.

Some of the most efficient engines on earth are enormous ship engines that use 2 stroke motors at low RPM with a boost stage.

A well engineered 2 stroke engine operating in a very narrow RPM band will run circles around the equivalent 4 stroke wide RPM band OHV car engine.

Mechanical to electrical conversion is crazy inefficient.
Try and guess the thermal energy of a nuclear power station for example (vs actual power it produces).

So measuring rotation power at a crankshaft and trying to get back to generator power is taking quite a large number of liberties with the conversions in between.
Why do you think cars don't run air conditioners off of the battery, it has nearly half the efficiency of crankshaft driven compressor.