With a single tank system the engine is modified to allow the engine to start and operate fuelled with 100% pure plant oil or when operating in extreme cold with a blend of PPO and diesel or petrol/gas.
The primary issue with a single tank system is achieving good starting performance and cold running. With single tank systems it is very important that the engine is in good working order. Before running an engine on biofuels it is advised to perform an engine health check to ensure that the engine is in good operating condition.
Single tank conversions often give good results on indirect injection engines using Bosch VE injection pumps or Japanese licensed copies. For other engines they should be viewed as extremely experimental
A small number of German companies have developed robust single tank conversions for some direct injection VW group Tdi engines.
Fuel system modifications
Due to the increased viscosity and differing combustion properties of PPO, compared to diesel fuel, benefits are gained by making a number of modifications to the fuel system to facilitate suitable fuel delivered to the engine. Excessive restrictions in the fuel system can cause insufficient fuel to flow to the engine or air to be drawn into the fuel system with associated rough running/engine stalling and possible damage to fuel injection equipment.
- Fuel hoses and pipes - Replacing fuel hoses and pipes in the fuel system with items with a larger internal diameter helps the fuel to flow freely.
- Fuel filter - Due to the higher fuel viscosity fuel filters cause a greater flow restriction and can prevent adequate flow to the engine particularly at start up when the fuel system parts and fuel is cold. Increasing the size of the fuel filter will reduce flow restriction and help to negate associated issues.
- Fuel Heaters - Heating the fuel reduces the viscosity of the fuel. Providing heat at, or before, the fuel filter will assist the fuel flow through the main bottle neck in the fuel system.
- Fuel injectors for PPO - Some designs of fuel injector are more suitable for running PPO.
- Fuel injection timing for PPO - A number of studies have found differences in fuel delivery timing and the combustion rate between PPO and petrodiesel. Modifications to the injection timing of the engine have proved to improve combustion when running PPO.
- Looped fuel return - Sending the returned fuel back to the fuel supply pipes/hoses rather than the fuel tank reduces the amount of fuel that has to be drawn from the fuel tank. The returned fuel tends to be at an elevated temperature an can give benefits in increasing the temperature of the incoming fuel. The use of a looped return can cause problems with air as the standard return-to-tank system allows air to be vented.
- Fuel lift pump - uprating or adding an electric lift pump will improve the flow of fuel to the engine. To avoid damage care has to be taken not to over pressure the fuel filter or injection pump.
Cold starting and running aids
Glow plug system
Engines fitted with combustion chamber glow plugs can have the glow plugs and associated control modified to provide improved performance for PPO. The majority of commercial and DIY single tank conversions use modifications to the glow plug system to provide improved starting and cold running performance with PPO.
- After glow system - Some engines are fitted, or can be retro-fitted with after glow systems to improve combustion during cold running. In these applications heavy duty glow plugs are used that can sustain being operated for dramatically increased periods. These systems also use glow plug controllers that modify the amount of pre and after glow in relation to either the engine coolant or engine bay temperature. Utilising an after glow system and modifying the control to provide extended glow periods has proven to give good results when fuelling with PPO. A number of different methods have been employed including manual control, modified after glow systems and custom built glow plug control.
- Longer glow plugs - a number of experiments with glow plugs that extend further into the combustion chamber have found that increasing their penetration into the combustion chamber by approximately 2mm provides improved starting performance. In some applications an alternative glow plug of suitable dimensions can be used, for other applications the glow plug seating is turned back on a lathe to provide the extended penetration.
Pre-heating
Some single tank systems use engine pre heating to aid starting and the cold running period. A number of different products are available to pre heat engines aimed at improved cold start and running for diesel engines in extremely cold temperatures with conventional diesel fuel. These heaters take a number of minutes, often half an hour or more, to raise the temperature of the considerable mass of the engine a significant amount. Some pre-heaters are available with timers, remote control or even telephone operated starting. Pre-heaters include:
- Mains electric heaters: the heater is plugged into mains electricity and pre heats the engine coolant. With some systems a heater is plumbed into the engine coolant hoses others fit a electric heating element into the coolant jacket of the engine.
- Diesel fired heaters: a diesel fired heater heats the engine coolant before the engine is started.
Cold running fuel delivery timing
Some engines employ systems that provide advanced fuel delivery timing at low engine speeds to provide improved combustion for cold starting and running. These systems can be modified to increase the period that they operate.
Air heating
Some engines use systems to heat the air before it is introduced to the engine cylinder. These systems are often used in engines without combustion chamber glow plugs to aid starting in low temperatures.
- Electrical air heaters - a matrix a heating wire in the air inlet is used to heat the air as it is drawn into the engine.
- Diesel flame start device - an electric heating element is used to ignite a small charge of diesel fuel that is introduced into the air inlet manifold. Flame start devices will not function reliably unless fuelled with conventional diesel fuel.
Notes
This article is focused on use with diesel engines with (unit-)direct fuel injection, yet Diesel engines with indirect injection are more suitable to run on plant oils, and even don't require any modification at all. In addition, although the heater mentioned is designed to run on a already present vehicle battery nothing prevents the user of adding a separate battery and running it from this if no battery is already present.