Off Grid Engine Projects

Currently we are building some 12 volt emergency generators and are simply a small Hyundai 212cc petrol engine costing £120 and small car alternators and fortunately we have several Bosch K1 units which are solid and robust and very long lived as well as being cheap and easily obtained.

First the alternators as these have certain specifications and the first is the output which is 120 – 140 amps which will charge single or multiple battery packs and the maximum alternator speed, which is the shaft speed of the alternator and not the driving engine speed, which is 8000RPM as this gives us the pulley sizes we require or the means to work them out. Next is the engine and this is a 6.5horsepower unit with a torque curve appropriate for our needs and has a decent reputation for reliability and comes complete as a compact unit and a 20mm diameter output shaft.

Next is our engine as this came with a set of drawings to mount the unit so the mounting plate was designed, next came the pulley assembly and as the engine had a 3600RPM maximum speed this gave a pulley ratio of 2:1 or two to one meaning the engine pulley must be twice the size of the alternator pulley as this gave the alternator shaft speed of 7200 RPM which is close to, but not exceeding the 8000 RPM maximum. I already had a lot of pulleys so pairs were chosen and these were the Poly V type which have a better contact area than traditional Vee belts and are less prone to slipping, and a crankshaft adaptor was turned to fit the crankshaft and drilled to accept the 4” pulley, this was bolted on, the crankshaft boss could be slid along the crankshaft tail and was mounted as close to the engine as possible to reduce bearing loadings and preserve engine life.
The alternator was prepared and the 2” pulley was bolted on and the unit was roughly mounted near to the engine and with everything measured and the mounting frame was designed which was made from 50 X 50 X 3mm box section and this was cut and welded to form our chassis and had the engine mounting plate welded on along with the alternator mounting and a mounting point for the pulley belt tensioner if required. A tensioner was made to prevent the alternator from moving as it was bolted rigidly to the frame, the belt was fitted and the tensioner adjusted to unsure it fitted correctly and a spring was fitted to the tensioner so it automatically tensioned and remained tensioned while the unit was running.

With everything running it was stopped and dismantled and the crankshaft adapter was removed and drilled above the keyway and tapped with a standard M6 thread and an M6 grub screw was fitted and everything was reassembled and the grub screw thread locked and tightened so everything was secure.

Next came the sides which were fabricated from 50 X 50 X 3mm channel and four uprights were made and the chassis was drilled and tapped M6 and the uprights were bolted on and a top plate was made which connected them all together and it was all drilled to accept M6 bolts and with the top plate fitted it was electrical connection time.

I designed a twin battery locking system which saw two batteries mounted to the plate and this was two bus bars for the + and – terminals and the terminals were split and the battery terminals slid into the terminal connectors, the free end was hinged and closed around the battery terminal and was clamped with finger screws. This meant you could disconnect one battery and remove it with the engine and alternator working and you could connect this to something else while your other battery charged, so you always had one battery connected to the charging bus bar so you didn’t blow the alternator, and we also had an inverter fitted and bolted to the bus bar.

Then came the wiring and with a car alternator you have to excite it for it to work and this is done by providing a small current to the exciting terminal and the control panel was made which involved a switch fed from the + bus bar and running to a 12 volt indicator light and onto the excitation terminal so with the switch on and the engine fired up the alternator would power the batteries and inverter. Next came the main cable connections and two holes were drilled into the top plate and rubber grommets were installed and the + cable from the screw terminal on the alternator was run through the grommet and cut to length and a lug was crimped and soldered on where it bolted to the + bus bar. The negative cable was bolted to the alternator housing and run through the other grommet where the second lug was crimped and soldered on and it was bolted to the – bus bar.

Connections were made to the invertor connections and these had lugs crimped on and were bolted to the bus bars and the unit was permanently connected as it had its own on/off switch on the unit.


Our next unit was a vertical unit which used a second hand engine from a lawn mower and I believe it was a Briggs and Stratton unit of 11 horsepower and it was a vertical engine with its shaft at the bottom and this meant the alternator was mounted underneath in the same way as out horizontal shaft engine and it took up very little floor space, but was taller. This unit actually ran four large leisure batteries which were used to power house 12 volt lighting and a small 12 volt fan for their log burner to blow heat around the house and their CCTV cameras and NVR which were all 12 volt, and their monitor which was converted to 12 volts.

This was set up differently as it was in an old coal house which had been cleaned out and painted and fitted with sound proofing, on the wall were various connections and these were:

The main house lighting 12 volt circuit
CCTV camera circuits
CCTV NVR unit and monitor
Outside 12 volt lights

They had wired their home exclusively for 12 volts in preparation for power cuts as this would give them lights, and the power for their log burner fan which blew the hot air around the house to keep it warm if they lit their log burner and they had included fuses for all the individual circuits located in their shed. They had installed outside 12 volt lights to light up from their house to the sheds where they stored their logs, and to act as a deterrent to thieves who obviously think its bonus time when the power is off, they also had a couple of lights inside the house which were permanently on during a power cut and ran from light sensors and switched on when it went dark.

They had switchable lights in their log shed so they could see the logs and get them and in the old coal house where the generator was located there were large 12 volt lights so they has light to see their generator and they could refuel or repair it as they could see it now.

Their CCTV systems ran from 12 volt transformers normally and they had relays fitted with mains 240 volt AC coils which held the contacts open and in the event of a power cut the mains power was lost and the coil was not energised and the contacts closed to make a circuit and it was switched to battery power automatically.

They had a connection for an invertor and this had a mains extension cable running from the invertor and into their fridge/freezer so they didn’t lose their food.


Unit number 3 was the larger 7hp engine and this had the same layout as the 6.5HP units and this meant 4 M8 holes at 60mm hole centres to mount things round the crankshaft so we made a mounting plate for an alternator to fit these holes and mounted the bracket to the engine and allowed the alternator to swing to adjust it and tension the belts. Being so close to the engine meant smaller belts so we used the smaller Vee belts and made it a twin belt set up which made it compact and it was set up to power remote batteries and a power supply was taken from the batteries and run through a switch to excite the alternator when it ran, along with the ignition light to show when it was charging and when it wasn’t and this powered a bank of standard batteries and the second bank through a split charger to power an inverter to provide mains power to the freezer and gas boiler.

Our final unit was a 15 HP unit and this ran a saw bench and this ran a blade speed of 2000 RPM and a 700 mm diameter blade was fitted which gave an 11” depth of cut and allows him to cut logs of up to over 20” in diameter by cutting one side and then the other and allows him to keep whole branches to season and just cut what he needs, and often what others may need.