Technical hype of LPG installers

Our manager started this company five years ago, already having a lot of experience in converting vehicles to LPG, general vehicle mechanical work, tuning petrol injection systems, building performance engines, teaching in subjects such as motor vehicle mechanics and digital / analogue electronics and as a freelance computer programmer. When the company was founded we only fitted systems from one manufacturer, whom we selected because through experience we knew they produced systems that were equally suited to low or very high powered vehicles, very reliable, and highly configurable to enable it to convert just about any vehicle. We still fit systems from this manufacturer, getting hold of spare parts continues to be absolutely no problem, any item arriving the next day and we continue to have a great working relationship with them. So why did we choose to also offer systems from a range of other manufacturers? Well, systems from this manufacturer are among the most expensive for us to buy in. There isn’t much profit margin in converting vehicles to LPG considering the amount of work and time that goes into each conversion to do a proper job. Competitors that fitted much cheaper to buy in LPG systems could undercut us by a large margin and we knew that even though they were charging around the same for a conversion as us, they were still making a lot more profit on each conversion we were. Most customers who enquire about a conversion are mostly interested in the bottom line - how much to convert my vehicle to LPG. Quality and other advantages can be a very hard thing to describe, particularly to potential customers of something like an LPG conversion - Names of manufacturers customers have probably never heard of before and technical selling talk.


To fully understand why a certain LPG system may be better suited to your vehicle than another LPG system you really need to understand how your engine and it’s fuel system work in detail. We’re not talking about the nuts and bolts of the engine or even the fuel injection system, we mean the intricacies of the engine design and how it is set up to run efficiently and smoothly, the sort of things that garage mechanics don’t consider because they never need to. Then you need to understand the different working methods employed by the different LPG systems, and a good deal of experience of working and installing them.  


If you contact most installation companies to enquire about converting your vehicle to run on LPG, chances are they will be biased towards fitting a certain system without properly explaining the options to you. Here are a few reasons why:


Some companies are the ‘sole importers’ for a certain manufacturer. Some manufacturers only make systems of a certain type (see Types Of Systems page). They probably don’t fit any other systems. If they do, then that should pose the question to customers why they should ever need to fit other systems besides the one that they will invariably sing the praises of. Then you may consider why they are the sole importer, the most likely reason is because they can buy that system at a cheaper price than any other system. The number of other installers that buy LPG systems from them is usually small, probably because when they mark the price of the system they import up to sell on for a profit, it becomes close to the price of systems which most installers in the know would rather fit for reasons such as being less problematic to install or more reliable.


Some companies, although free to choose from any suppliers and manufacturer’s systems, only install systems from a certain manufacturer. Although we have always worked with a large range of LPG systems, we too used to only fit systems from one manufacturer. But then, we genuinely believed and still do, that it is a good system. We have always kept a watchful eye on the wider LPG systems market and tested other systems. If an installer only install a certain system then it could be a decent quality one, but just as easily be a poor one. It is easy for installers to be stuck in a ‘comfort zone’, used to a certain system and not bothering to learn a new one. They may also have fallen for some of the manufacturer’s hype, the system might be cheap to buy in, or by sticking to that manufacturer they may be able to obtain it for marginally better prices.


Here’s a common quote - ‘We are members of the LPGA, even if you don’t come to us you should ensure you have your vehicle converted by an LPGA registered company or you may not be able to get insurance or the installation might not be recognised as a legitimate one, etc etc’. The LPGA is only a trade body, not a government setup. The fact is that there is no legal requirement for installers to be members of the LPGA. It isn’t hard to become a member, you have to pay them a substantial sum, they inspect your premises and a few vehicles you have converted to ensure they meet their code of practice. The code of practice has nothing to do with the quality of the parts fitted and takes little account of how well the vehicle drives after conversion. It is mainly concerned with safety but the points it covers are generally common sense. It is easily possible to do a low quality job of installation and still comply with the LPGA’s code of practice. We don't have any plans to become members of the LPGA because we think there is already too much red tape in business without private individuals inventing more (for their own financial gain), and we would have to pass on the costs of being a member to our customers. The law states that a vehicle LPG fuel system must be installed by competent persons. We are endorsed by many manufacturers as competent persons and have certificates issued by manufacturers to prove we are authorised and approved installers. We certainly should be, considering the amount of vehicles we have converted! There are a few insurance companies that do require you to have your LPG system converted by an LPGA registered installer, the vast majority do not. The minority that do are usually not the cheapest anyway. In five years only three customers have said they have had a problem. One was a taxi driver, it was not his insurance company but his particular local council that required his taxi to have an LPGA certificate. We got him an LPGA certificate for his taxi two days later, no problem. One of the others changed his insurance company and saved a lot of money on his insurance, and the last one’s insurance company was happy to insure him but he was worried about future insurance because of the message that the LPGA puts out.


Let’s look at some of the things installers say:


‘A very high quality system’

Quality can be very subjective and difficult to describe. If a person who had no prior knowledge of LPG systems were to compare several different systems, what aspects might they consider might make one system of a higher quality than another? There would be first impressions. Systems that look bigger, more heavy duty than others or more substantially built can give the impression of quality to some people, on the other hand to others they might give the impression of ‘barn door engineering’. There might be impressive on paper specifications, such as a system that is capable of powering high powered vehicles, has fast working and high flowing injectors or an electronic system which has a computer that can do a lot of calculations per second. There may be claims from some of the manufacturers of the systems, such as our system integrates better with modern vehicles because... etc etc, or the injectors of our system are unique and better than others because... etc etc. Claims might include reliability statistics, or tell you that the system is very popular (somewhere in the world), so it must be a good system for so many people to choose it.

What attributes should a good quality system have to us? Components that are substantially built are better because they are less likely to be damaged in the harsh place that is the engine compartment.

Pressure reducers (vaporisers) that can flow a lot of gas allow them to power high powered vehicles or work well within their capabilities on average power vehicles. Pressure reducers that give fairly constant pressures usually mean the brain of the system doesn’t have to compensate too much for pressure differences and can help make the fuelling more accurate.

LPG injectors that we know work accurately, because an inaccurate injector may effectively open and close at differing speeds and have the effect of making the fuelling less accurate. LPG injectors that are quiet, because some can be fairly loud. LPG injectors that are reliable, because some are very reliable and some simply are not. Injectors that are on a rail (block) because this means each individual injector will get the same pressure of LPG and hence all the injectors will deliver the same amount of LPG for a given injector open time from the LPG brain. We prefer injectors that are not designed to mount directly onto the inlet manifold, this may seem counter intuitive because the best place to have the LPG enter the engine is as close to the engine’s petrol injectors or even inlet valves as possible. In practice injectors that mount directly onto the engine inlet manifold actually inject gas further away from the ideal position than remote mounted injectors because their size means positioning them close to the ideal position can be impossible. On remote injectors the metering jets are on the injector itself, as with direct mount injectors, but the metering jets then connect to a pipe which leads to a very small injector that can be mounted extremely close to the cylinder head on the inlet manifold. On some vehicles, because of the design of the inlet manifold, placing even these very small injectors in the ideal position can be impossible, in which case we use ’straw’ type injectors. Straw type injectors are, as the name suggests, shaped like drinking straws but they are made out of copper which can be bent into any shape.  We can run them inside the inlet manifold so the tips can actually lead right inside the inlet port of the cylinder head and get even closer to the inlet valve than the petrol injectors if we so wish. With direct manifold mount injectors this would not be possible,  making doing a good job of converting some engines impossible with direct mount injectors and compromising on injector position with most others.

The ECU (electronic control unit or brain of the system) should be well made, having a strong body preferably made out of metal. Most systems use a multi-plug on the wiring harness so the ECU can quickly be removed without cutting any wires. The multi-plug should also be well made, watertight and the whole ECU should give us the impression it can withstand the very high / cold temperatures and sometimes damp or oily conditions under the vehicle bonnet. Then it gets more complicated because the next thing we need to consider is the strategies used by the ECU to control the LPG injectors and how successful these strategies are on various different engines. All sequential system ECUs that are suitable for use on modern fuel injected vehicles are designed to achieve the same thing. They electronically cut the signal between the petrol ECU and the petrol injectors when running on LPG, so that the petrol injectors stop working and you don’t use any petrol. They monitor the same signal, timing it to see how long the petrol injection pulses are. They take this petrol injection timing figure, apply a mathematical algorithm and calibration information set by the installer to work out the correct amount of time to hold the LPG injectors open for. The major differences between systems is in the way these algorithms work, whether they can calculate individual length of opening figures for each LPG injector or give each LPG injector the same opening time for each revolution of the engine (remember that sequential systems all operate injectors individually one after the other in a sequence, hence sequential), and the way they are calculated. Many of the better known systems all use ECUs manufactured by the same electronics company and look very similar but there are internal differences in the circuitry that drives the LPG injectors, the interface with the vehicles petrol ECU (to measure the would-be petrol injection times) and the programming that allows the installer to calibrate them. The better known systems generally measure only the injector open length signal for the first petrol injector on each bank of cylinders and after doing the calculations give each LPG injector on that bank of cylinders the same length of injector open time for that revolution of the engine. They do not wait until they receive an injector open signal for each petrol injector in turn before starting to inject LPG for each individual LPG injector in turn, instead they space out the time that each LPG injector starts to open based on engine RPM. Some of the less well known or newer systems calculate individual lengths of times for each of the LPG injectors to be open on each bank of cylinders and will wait until they receive a signal from the petrol ECU that it is trying to open a petrol injector before opening the appropriate LPG injector. So then, given this information you might think it is clear cut, that one of the latter type of systems is the clear winner here. But there is much more to consider. Most LPG ECU’s have a similar amount of processing power to most others and all systems tend to make good use of the processing power available to them. You can understand that if you have a four cylinder engine, a system that is trying to calculate injection times for each individual cylinder would need to do four times as many calculations as a system that only measures the injection times for one cylinder on that bank of cylinders. Considering that some of the computers in the ECU’s of the latter type (that do calculate individual injector opening times) are only about the same or less powerful than the better known systems that only measure injection times from one cylinder, you may begin to wander how they manage to do four times as many calculations. The answer is that they are not capable of doing four times as many calculations. They are able to individually time each injector by making a huge compromise with the algorithms that convert petrol injection times to appropriate LPG injection times and the level of control the installer has over the calibration.  The better known systems are usually ‘mappable’, they have a fuel calibration ‘map’. In fuel injection terms a map is like a computer spreadsheet with RPM on the X axis and engine load on the Y axis, so on a system that has a map with 20 points on the X axis and 20 points on the Y axis there would be 400 boxes which we could adjust. If your car has electronic fuel injection, the petrol injection ECU has a map. It is the single most important point of reference the petrol system uses to determine how much fuel to inject for any RPM and engine load.  So the map is the most important reference, but the petrol system also applies further calibration information which it has self learned by monitoring the exhaust emissions over the long term. The long term information is called the slow fuel trim and there may be several long term fuel trims, each covering different areas of the map. So let’s suppose you’ve just accelerated on a road to 50mph and now you back off the throttle a little to hold a cruise of 50mph. You might just have reached an engine speed of around 2100rpm and have your foot on the accelerator about a quarter of the way. So what is going on in the petrol ECU? It looks up the figures in it’s map for 2100rpm and a quarter throttle. The map might contain information that under these conditions it should open the petrol injectors for, say, 6 milliseconds per engine revolution. Then it looks at the slow fuel trim that applies to this part of the map. Suppose the fuel trim was 5% for this part of the map, it would add 5% to the 6 milliseconds it has looked up on the map and actually open the petrol injectors for 6.3 milliseconds per engine revolution. So we’re doing 2100rpm with a quarter throttle and injecting 6.3 milliseconds per engine revolution. It continuously monitors the exhaust emissions by using a sensor (Lambda probe) placed in the exhaust which measures the oxygen content of the exhaust gas flowing past it. The Lambda probe gives a varying voltage or electrical resistance directly related to how rich or lean the engine is running. The ideal mixture for burning petrol in air is around 14.7 parts air to 1 part fuel but a mixture down to around 13.9:1 can give slightly more power at the expense of higher HC (hydrocarbon - unburned fuel) and CO (carbon monoxide - partially unburned fuel) emissions and less economy for the given amount of air, while a mixture up to 15.4:1 can give slightly better economy at the expense of power and sometimes NO (nitrous oxide - nitrogen from the air chemically combined with oxygen from the air because of very high temperatures during combustion) emissions. Back to the injection system, it uses the information from the Lambda probe to check that the amount of fuel it is injecting is correct, not too lean and not too rich. Let’s say the Lambda probe sends back information that the engine is running too lean, the injection system will immediately starts to compensate by applying another calibration factor, the fast fuel trim. Unlike the slow fuel trim, which is learned over a period of time, the fast fuel trim works on a spur of the moment basis and changes quite rapidly. So now we have the 6 milliseconds from the fuel map, the 5% slow fuel trim takes this to 6.3 milliseconds, and a fast fuel trim of, well let’s just say 1% for now. So it adds 1% to the 6.3 milliseconds and comes up with 6.363 milliseconds. In practice it will probably round this to 6.4 milliseconds. Now, monitoring of the Lambda sensor is an ongoing process. If the engine is still running lean the fast fuel trim will continue to rise until the Lambda probe gives the reading that the mixture is correct. In actual fact the mixture will go slightly too rich, which will be reflected by the fast fuel trim figure being lowered slightly until the engine goes slightly too lean, and the engine will continue to fluctuate very quickly between a very slightly lean and a very slightly rich mixture It will continue to go slightly rich then slightly lean, so the fast fuel trim and Lambda probe readings will fluctuate very quickly but stay. Over time, the fast fuel trim effects the slow fuel trim which is stored in memory, so next time the engine has to work at 2100rpm with a quarter throttle it will probably be closer to the ideal mixture to start with before even applying a fast fuel trim. 



‘This system is the latest available’

‘This system is the most up to date available’

‘Latest generation’

Any sequential system can be loosely described this way because sequential systems were the most recent major design shift to keep up with modern vehicles advanced electronic engine control systems, following on from the older multipoint non sequential type. The actual system they are talking about may have been out years. Actually if it is a well known system that has been around longer than most it could be a better bet than a system that hasn’t been around for very long. If it is a very new system, has it had time to prove itself as reliable? There are exceptions to rules, and most installers are aware of quirks when installing systems they know on certain vehicles, so have the installers had time to discover these quirks? The comments by themselves say nothing about the quality of the system and doesn’t describe anything about the strategies it uses to ensure the fuelling is correct and integration with the petrol system is optimised.


‘The most advanced available’. Why is it? Installers that say this are usually just repeating the selling points claimed by the manufacturer of that system and probably don’t really understand what they are saying or the possible advantages gained even if they were true. Let’s suppose you were starting from scratch developing ‘the most advanced LPG system’. What aspects of the system would be most important in making it the most advanced? It would certainly be a sequential system, but what else? To be the definitive most advanced system, to us, would mean it had a reducer which could flow the most gas at any given pressure whilst being the smallest around and the most reliable. It would have the best injectors that never wore out, were extremely quiet, opened the quickest, flowed the most gas when open and at the same time be very small. The brain of the system would offer a range of strategies, all aspects of how it works would be completely configurable and again it would be reliable. Such a system doesn’t exist! Different sequential systems all work in the same general way, but use different strategies to integrate properly with existing vehicle systems and ensure the correct amount of gas is injected. We regard some of the strategies employed to be shortcuts which have partly allowed the systems to employ them to be produced cheaper. It isn’t clear cut as to which LPG system will integrate best with a certain model of vehicle, but there are cases where a certain system will do a good job where another system will do a great job and yet another system will not work properly.


‘This system is the most advanced because it uses real sequential injection’. All sequential systems use real sequential injection, so what are they trying to say by this comment? Well, many systems, including some of those which we regard to be of higher than average quality, measure only the amount of time that the first petrol injector on each bank of cylinders would be open for (if not disconnected by the LPG system). So on an average four cylinder engine it only measures the time that number one cylinder’s petrol injector would be open on each revolution of the engine and on a V8 it will probably measure number one cylinder’s injection time and number fives cylinder’s injection time. Then, most systems will calculate (using various methods and calibration techniques) the amount of time that the LPG injectors should be open for. Now, most systems will give each LPG injector on that bank of cylinders this same injection time and time the opening of each LPG injector based on engine RPM. ‘Real sequential injection’ systems measure each petrol injectors would be opening times individually and calculate the time for each LPG injector to be open separately. The part of the ’advantage’ that installers of these types of systems try to sell their systems on is that they also start opening each LPG injector at the same time as the petrol injector would open. These points seem like reasonable arguments, indeed the first point can be a good point about these systems on a small minority of vehicles. But there are serious shortfalls which are not apparent unless you understand a little bit about why a petrol injection system works the way it does, how other LPG systems work, and how these types of system are calibrated and installed. This is what they either don’t understand themselves or choose to ignore when doing the selling talk on these systems.