Fuel Pump Timing: Crucial for Engine Performance

The Importance of Fuel Pump Timing

Have you ever noticed some vehicles emitting a noticeable blackish exhaust while others don't? The colour and density of exhaust gases are significantly influenced by fuel atomisation and, crucially, fuel pump timing. When fuel is injected at the precisely correct moment, it leads to complete combustion, resulting in clean exhaust gases. Conversely, any deviation in this injection timing can cause incomplete combustion, leading to dark smoke laden with soot and unburnt particles. If left unchecked, improper fuel pump timing can lead not only to smoky exhausts but also to excessively high exhaust temperatures, high-temperature corrosion, and generally abnormal engine running conditions. Essentially, fuel pump timing refers to the specific moment at which fuel is injected into the engine cylinder. Ideally, this timing falls between 10-20 degrees BTDC (Before Top Dead Centre) and 10-20 degrees ATDC (After Top Dead Centre), where TDC represents the uppermost position a piston can reach within the cylinder.

To truly grasp why and how to check fuel pump timing in both two-stroke and four-stroke engines, it's essential to understand timing diagrams and the fundamental working principles of fuel pumps in internal combustion (IC) engines. A timing diagram graphically illustrates the six key events in the fuel combustion cycle for both engine types. For a four-stroke engine, a timing diagram typically outlines aspiration, compression, fuel injection, expansion, exhaust, and a brief overlap period. In contrast, a two-stroke engine's timing diagram depicts scavenging, the post-scavenge period, compression, fuel injection, expansion, and exhaust.

Fuel pumps can be controlled either manually or electronically, as seen in modern ECU-based common rail injection systems. Regardless of the type, the fuel pump's primary role is to deliver a predetermined amount of fuel at the desired pressure into the cylinder. Let's delve deeper into the necessity of correct timing and its profound effects on engine operation and performance.

Factors Determining Changes in Basic Fuel Pump Timing and Calibration

Several factors can necessitate adjustments to your fuel pump's timing. Understanding these is key to maintaining optimal engine health.

1. Timing Diagrams: The Blueprint for Combustion

A timing diagram serves as a graphical representation of the engine's operational events, including the opening and closing of exhaust and inlet valves, alongside the precise point of fuel injection. This diagram maps the entire combustion process over the 360-degree rotation of the crankshaft, essentially charting the piston's movement from TDC to BDC. By comparing the engine's actual performance against its timing diagram, mechanics can ascertain if the engine is operating in sync with the manufacturer's recommendations. Any deviation from the specified injection timing can have detrimental effects on engine performance. In the worst-case scenario, particularly in two-stroke engines, incorrect timing can even lead to the engine running in reverse. This is plausible because two-stroke engines often share identical timing for scavenging and exhaust, eliminating the need for precisely sequenced valve operations to complete the combustion process. Therefore, adhering strictly to the manufacturer-provided timing diagram is paramount.

Here's a look at the typical timing diagram for a two-stroke diesel engine:

Combustion by-products in the form of exhaust gas are expelled into the atmosphere through the exhaust port, typically between 110-120 degrees ATDC and 110-150 degrees BTDC. Concurrently, fresh air is drawn into the cylinder via the scavenging process, occurring between 130-150 degrees ATDC and 130-150 degrees BTDC. This fresh air is then compressed within the cylinder, with fuel injection occurring between 10-20 degrees BTDC and 10-20 degrees ATDC. The combustion of the fuel-air mixture causes a rapid expansion, forcing the piston downwards, and the cycle repeats.

Similarly, in a four-stroke engine, the aspiration phase is completed with the closing of the inlet valves around 145-155 degrees BTDC. Fresh air is subsequently compressed within the cylinder, followed by fuel injection between 10-20 degrees BTDC and 10-20 degrees ATDC. The resulting combustion causes a sudden expansion, pushing the piston downwards. The exhaust valve opens around 120-150 degrees ATDC, and finally closes at 50-60 degrees ATDC. There's an overlap period that begins around 70-80 degrees BTDC with the opening of the inlet valve.

2. The Working of Fuel Pumps in IC Engines

A fuel pump is designed to deliver a specific quantity of fuel to the combustion chamber at regular, fixed time intervals, irrespective of the engine's load. The pump generates high pressure, which is essential for injecting fuel into the cylinder against the resistance of the fuel injector's spring. This high pressure aids in atomising the fuel within the cylinder, ensuring proper penetration and maintaining high combustion efficiency. Atomisation, in essence, is the process of breaking down fuel into extremely fine particles to facilitate better mixing with the air.

There are three primary types of fuel pumps used in internal combustion engines: the simple mechanical pump, the plunger pump, and the common rail injection system with electronically controlled fuel injection. Simple mechanical pumps were common in older automobiles, supplying fuel to the carburetor. The carburetor's role in a Spark Ignition (SI) engine is to create an air-fuel mixture at a specific ratio outside the combustion chamber. This external mixing process adds to the overall time required for combustion, thereby affecting the entire engine cycle. Consequently, carburetors have largely been replaced by superior electrical and mechanical plunger-type fuel pumps.

An example of a plunger-type mechanical pump is the Bosch jerk pump. Electronic fuel pumps are integral to common rail injection systems. We've explored both the Bosch jerk pump and common rail injection systems, along with fuel injectors, in more detail previously.

A. Bosch Jerk Pump

The Bosch jerk pump is renowned for its simple yet effective design, featuring a single piston assembly, known as a plunger, that operates in conjunction with a cam profile. A helical spring mounted on top ensures continuous contact between the plunger and the cam follower. The plunger moves vertically within the cylinder barrel, drawing in fuel oil and then injecting it at high pressure. A precisely machined helix on the plunger allows for control over the amount of fuel injected by altering the helix angle as the plunger rotates. The plunger's rotation relative to the barrel is managed by a rack and pinion mechanism. To adjust the fuel pump timing, one must manually alter the relative height of the plunger. Increasing its relative height advances the injection timing, while decreasing it retards the timing. For engines without Variable Injection Timing (VIT), fuel pump timing can be adjusted by modifying the pump shims on the base surface.

B. Electronically Controlled Fuel Injection (Common Rail Injection System)

Unlike traditional mechanical injection systems, a common rail injection system utilises a central high-pressure fuel pump for each cylinder. These high-pressure pumps are typically driven mechanically by the engine's movement; however, newer designs incorporate electrically driven pumps for smaller automotive engines. While diesel engines commonly employ common rail injection systems, petrol engines utilise gasoline direct injection. The commonality lies in the fact that fuel injection in both systems is electronically managed by a Central Control Unit (ECU). A common rail system uses a pair of central fuel pumps to deliver fuel oil to the common rail, which typically operates under pressures ranging from 1000 to 1800 bar, although this can vary between engines. The ECU receives signals from various sources, including air temperature, crank angle, engine speed, load, jacket water temperature, and fuel pressure, to maintain accurate fuel pump timing. Therefore, the ECU of a common rail injection system can be programmed to modify injection timing. This offers the flexibility to change the entire ECU or specific components rather than sending the unit for reprogramming to adjust fuel pump timing.

Effects of Fuel Pump Timing on IC Engines

Why is it so important to ensure your engine's fuel pump timing aligns with the manufacturer's recommendations? The timing of fuel injection can be influenced by various factors, including the fuel pump's timing itself, faulty or broken injector springs, and the quality of the fuel used. Any deviation in fuel injection timing can lead to adverse effects on the engine, ranging from heavy vibration and shock loads to energy loss, incomplete combustion, and elevated exhaust temperatures. Other visible symptoms might include difficulty starting, black smoke emission, unusual engine behaviour, engine misfires, and a sudden increase in fuel consumption. If left unaddressed, it not only contributes to pollution and potential overheating of engine components due to hot corrosion but can also subject the engine to unsafe stress levels, increasing the risk of unintentional reverse rotation. This is precisely why checking fuel pump timing is a standard practice whenever any deviation in fuel injection timing is suspected.

Effects of Injection Timing on Engine Performance:

For a typical IC engine, the emission of Nitrogen Oxides (NOx) increases significantly with early ignition, meaning fuel is injected earlier in the cycle (BTDC). Similarly, Sulphur Oxides (SOx) emissions also tend to rise slightly after approximately 20-26 degrees BTDC (too early ignition), which is primarily dependent on the sulphur content of the fuel. Conversely, the generation of hydrocarbons (HC) by the engine decreases as ignition timing advances; earlier ignition results in lower hydrocarbon generation.

Engine peak pressure and exhaust gas temperature typically react in opposite ways to injection timing. Exhaust temperature increases as injection timing is retarded, while peak pressure increases when injection timing is advanced. Likewise, exhaust gas temperature gradually rises from early ignition towards late ignition due to changes in fuel pump timing. A small degree of change in fuel injection timing can result in a sudden surge in peak pressure.

There's no doubt that injecting fuel earlier into the cylinder can boost power, improve efficiency, and increase thrust, but this comes at the cost of increased shock load, higher vibration, and elevated NOx emissions. On the other hand, retarding injection timing can diminish engine performance, increase exhaust gas temperature, and make engine operation less economical. Therefore, it is crucial to maintain injection timing within the limits recommended by the engine manufacturer.

How to Check for Incorrect Injection Timing

The most straightforward and effective method to identify potential deviations in fuel pump timing is by taking a draw card diagram. To do this, an indicator instrument is attached to the indicator cock, with the indicator drum set 90 degrees out of phase with the piston stroke. This process helps measure pressure changes throughout the complete combustion cycle, allowing for the detection of any deviations in compression and maximum pressure caused by faults. Compare the obtained draw card diagram with a normal or manufacturer-recommended diagram. Any sudden peaks in maximum pressure or slight increases in compression pressure, accompanied by a delayed and smaller spike in maximum pressure, are indicators of either early or late fuel injection. Once a deviation is identified, confirm it using the 'hairline' method. The hairline test involves checking fuel pump timing by observing the crank angle at which the marking on the plunger aligns with the marking on its body. When the unit is at TDC with both markings coinciding, the crank angle marking on the flywheel will indicate the precise fuel injection position on the timing diagram. For smaller, single-cylinder engines, a similar check can be performed using the timing pin method. Remove the timing pin bolt cover and rotate the crankshaft until the timing pin engages with the notch on the camshaft for the fuel pump. Then, insert a threaded bolt into the pinhole to check if it slides in smoothly. If the bolt enters the pinhole without resistance, the fuel injection timing is correct; otherwise, corrective adjustments are necessary.

How Can We Adjust Fuel Pump Timing?

Once you've identified incorrect fuel pump timing, rectifying it is generally straightforward. Fuel pump timing can be adjusted by altering the number of shims fitted to the pump base. Adding or removing the required shims will set the desired injection timing. For some engines, it may also be necessary to manually adjust the relative position of the cam concerning the crankshaft to achieve the correct timing. As a general guideline, it's recommended to set the fuel pump to initiate injection at 15 degrees BTDC.

For engines equipped with VIT (Variable Injection Timing), the fuel pump timing can be easily adjusted by modifying the relative height of the plunger within the barrel. Lowering the plunger retards the injection timing, while increasing its height or relative position advances it. Alternatively, you can employ the traditional method of changing the cam's position relative to its shaft. Rotating the cam in the direction of normal camshaft rotation will advance the injection timing, while rotating it in the opposite direction will retard it.

Correcting incorrect injection timing in electronically controlled systems, such as common rail systems, can be achieved through reprogramming or recalibration of the ECU. In industrial settings with large engines, pre-programmed chips are often available, allowing for a simple replacement of the existing chip if there are doubts or confusion. However, typically, a reset or recalibration of the ECU is sufficient to resolve timing issues without the need for complete replacement or reprogramming.

Note: This article was produced at the request of Russel Devotta.

Also Read:

• Do You Know We Write Post On Your Request?
• Request your own Topic!

If you want to read more articles similar to Fuel Pump Timing: Crucial for Engine Performance, you can visit the Automotive category.