Combustion and emissions characterization of terpenes with a view to their biological production in cyanobacteria

In developing future fuels there is an opportunity to make use of advances in many fields of science and engineering to ensure that such fuels are sustainable in both production and utilization. One such advance is the use of synthetic biology to re-engineer photosynthetic micro-organisms such that they are able to produce novel hydrocarbons directly from CO₂. Terpenes are a class of hydrocarbons that can be produced biologically and have potential as liquid transport fuels. This paper presents experimental studies on a compression ignition engine and spark ignition engine in which the combustion and emissions of 12 different terpenes that could potentially be produced by cyanobacteria were assessed as single components and blends with fossil diesel and fossil gasoline. The 12 terpenes were chosen to explore how small changes to the molecular structure of geraniol (a terpene most easily produced by cyanobacteria) impact on combustion and emissions. Furthermore, the toxicity of some of the best performing terpenes were assessed using the model cyanobacterium Synechocystis sp. PCC6803 (hereafter, Synechocystis) as a prelude to a metabolic engineering programme. The compression ignition engine tests were carried out at constant injection timing and constant ignition timing, and the spark ignition engine tests were conducted at a constant spark timing and a constant lambda value of 1. Of the terpenes tested in the compression ignition engine, geranial and farnesene were found to be the best performing single component fuels in terms of combustion and emissions. In blends with fossil diesel, the presence of geranial or farnesene did not have a significant effect on combustion phasing up to a terpene content of 20% (wt/wt), though levels of NOx and CO did increase. In the spark ignition engine experiments of terpene and fossil gasoline blends, citronellene and linalool were found to be soluble in fossil gasoline and combusted in a steady manner up to a terpene content of 45% and 65% (wt/wt) respectively. Of those terpenes with the most potential as either diesel or gasoline fuels, geraniol and geranial were found to be the most toxic to Synechocystis, with farnesene and linalool less toxic and citronellene having no detrimental effect. Addition of n-dodecane to the cultures was found to ameliorate the toxic effects of all five terpenes.