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My wife's 2003 Honda Jazz has dual ignition, as I found out when questioning the service manager about the account for 8 new spark plugs. What is the reason for dual ignition in a little 1.3lt engine ? Are there any other standard automotive engines that use dual ignition ?

Honda’s idea behind the dual ignition design (two spark plugs per cylinder) was to reduce both fuel consumption and tail pipe emissions in comparison to a standard 8-valve or a competitors multi valve engine . This is achieved by altering the firing intervals within the combustion chamber in an attempt to achieve complete combustion under different conditions (engine load and speed for example). It also produces improved low down torque which is just as important as power in any engine design.

It is almost a cheaper version of Honda’s claimed Variable Valve technology (V-TEC) which requires a more complicated multi valve cylinder head design.

In comparison to other 1.3 litre engines, the claimed fuel consumption benefits can outweigh the cost of the extra four spark plugs over the 40,000km life span these plugs have. As an example, if you saved one litre of fuel per 100km’s over 40,000kms the estimated $180 to change the spark plugs (parts and labour) is still a saving. The additional cost is in fact only $90 as 40K is a fairly standard service replacement interval for many manufactures whose engines run standard spark plugs.

The potential downside of this engine design is the positioning of the ‘extra’ spark plugs with some non Honda service outlets not realising they are working on a dual ignition engine and end up replacing four plugs only. This can led to many return visits to the repairer for rough running issues.

If you were a diehard Honda fan it would be easy to accept this technology as being financially better on the wallet long term however, adding an additional $90 onto a 40K service can be a unpleasant surprise when it comes time to pay the bill.

The later model Jazz engines have reverted back to a more conventional one plug per cylinder design with long life plugs fitted (change every 100K).

Very few manufacturers have used the dual ignition technology in recent times.

Thank you for the explanation.
One clarification, please. When you say 'altering the firing intervals' do you mean that the two plugs in a cylinder fire at different times ?

Words below are Honda NZ's technical explanation of the firing process.

Each pair of spark plugs is fired sequentially with the interval between the two depending on engine rpm and load.

The intake side plug ignites first, then as the flame propagates, the exhaust side plug is fired (before top-dead-centre), and the flame expands rapidly into the whole area to achieve complete combustion.

This arrangement provides much faster combustion and higher cylinder pressures to provide high engine output.

The programming of the ignition timing map achieves a careful balance between economy and power output. At large throttle openings, up to engine speeds of around 2600rpm, intake side ignition is advanced and the exhaust side slightly retarded. In the mid-range the exhaust side is further retarded, optimising output.

At high engine speeds ignition is almost simultaneous for optimum power and torque. At part-throttle, the simultaneous ignition phase is more extensive, commencing as low as 3500 rpm.

Many thanks for all the info on dual plugs.
The modern car seems to have reduced 'owner maintenance' to cleaning the windscreen and blowing up the tyres !!!!