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[abridged] The LMXB 4U 0614+091 is a source of sporadic thermonuclear (typeI) X-ray bursts. We find bursts with a wide variety of characteristics inserendipitous wide-field X-ray observations by EURECA/WATCH, RXTE/ASM,BeppoSAX/WFC, HETE-2/FREGATE, INTEGRAL/IBIS/ISGRI, and Swift/BAT, as well aspointed observations by RXTE/PCA and HEXTE. Most of them reach a peak flux of~15 Crab, but a few only reach a peak flux below a Crab. One of the burstsshows a very strong photospheric radius-expansion phase. This allows us toevaluate the distance to the source: 3.2 kpc. The burst durations are between10 sec to 5 min. However, after one of the intermediate-duration bursts, afaint tail is seen to at least ~2.4 hours after the start of the burst. Onevery long burst lasted for several hours. This superburst candidate wasfollowed by a normal type-I burst only 19 days later. This is, to ourknowledge, the shortest burst-quench time among the superbursters. A superburstin this system is difficult to reconcile if it accretes at ~1% L_Edd. Theintermediate-duration bursts occurred when 4U 0614+091's persistent emissionwas lowest and calm, and when bursts were infrequent (on average one everymonth to 3 months). The average burst rate increased significantly after thisperiod. The maximum average burst recurrence rate is once every week to 2weeks. The burst behaviour may be partly understood if there is at least anappreciable amount of helium present in the accreted material from the donorstar. If the system is an ultra-compact X-ray binary with a CO white-dwarfdonor, as has been suggested, this is unexpected. If the bursts are powered byhelium, we find that the energy production per accumulated mass is about 2.5times less than expected for pure helium matter.

What ignites on the neutron star of 4U 0614+091?